Alright gonna break this down by character and further into categories for each because there is a lot. Feel free to give me a poke if you want more details on any of these!
Sion
Magic: Let's start with the easy one. Sion's world has magic, and Sion is capable of using it! The way it works is, there's tiny motes of... something floating around the world. Normally these are invisible to humans, but someone who's undergone the right mental training can see them. And then manipulate them. Through a combination of physically interacting with and rearranging the motes into a pattern (usually with a hand/finger) and speaking an appropriate incantation, a spell can be cast! (To a non-magical observer this looks like you're creating a glowing diagram with your finger in midair, fun!) Spells include things like fire, a blast of water, temporarily deadening sound in an area, etc. Sion only knows Roland's spells, though in theory it'd be possible for him to learn others given the opportunity and sufficient time.
If you choose this option, feel free to ask me for info on incantations or such if you want! Or just have your character be puzzled by the floating glowing motes they now see everywhere, whatever works.
The Mad Hero: Now for the tricky one. This is a partial possession by/fusion with one of the gods of Sion's world, namely Aslude Roland aka the Mad/Fallen/Black Hero. It's basically a big package deal with multiple abilities and effects as follows!
Anti-existence black swords These are literally the ability to slay gods, but the swords made of darkness can be summoned at will and used against pretty much anything. Things destroyed by the swords are effectively erased from existence and thus wiped from human memory, though your mileage may vary on that here in pokemon land. Note too that using this power comes with a price: every time you do, you'll become slightly less human and more, well, demigod-ish. Which will be elaborated on below.
Perception of the spiritual plane ... Look, you're basically now empowered by a god, of course you can perceive the dimension/plane/whatever you wanna call it that such beings normally exist on. That said there's two noteworthy drawbacks: first, that if you're not used to seeing/being in two places at once, it's hard to actually focus on more than one at a time. Second, and more importantly, while this lets you interact with other spiritual beings so you can kill them with your swords of darkness that also means that said other beings can now interact with and harm YOU, should they be so inclined. Think of it like humans being Normal types and gods being Ghost types so they're normally mutually immune; in exchange for being able to hit ghosts you now lose your normal immunity to them.
Becoming a demigod/endless agony This is a twofer. Since you're fused with a god and are (presumably) slowly assimilating his powers, you're not longer fully human yourself! Plus, you can accelerate this process by doing things like actively tapping into this power a lot. Push this process far enough along and you'll get nifty(?) side effects like glowing gold magical god blood! However, at ALL stages this comes with it incredible, unending agony. It feels like literally every individual cell in your body is somehow, impossibly, in pain. Nothing stops or dulls it, and it persists even if you're unconscious. The only way to block it is to block all the Mad Hero's power from you, rendering you normal human again.
Possession Of course, the thing with trying to absorb a mad god's power? Said mad god tends to come with it. So enjoy having that presence in your head, constantly trying to take over and subsume your consciousness into his own. The Mad Hero basically wants to first break free of his prison (which... this place is NOT said prison, so that's a little moot right now) and second go finish killing all the gods. Humans are largely beneath his notice beyond how he can possibly use/get power out of them. The only being he MIGHT possibly care about (and that mostly from being reincarnated as a human countless times slowly changing him) is the Lonesome Demon, but since only half of the Demon is here there's not much he can do about that either. So really have fun with a very frustrated mad god wanting to rampage without anything to rampage at I guess.
Shrike
Fayblood: Shrike was sired by a fay, which means he's a human with a few... extra things tacked on. In his case this means he has somewhat funny-shaped ears, his finger- and toenails are narrower and thicker than a normal humans (think halfway toward being claws), and his whole body is covered with markings that vaguely resemble an animal's spotted coat. He's also unnaturally strong, capable of things like hauling a full-grown man wearing armor out of a flooding river, has a slight innate ability to kind of just sense magic, and technically has access to basic standard fay abilities (like casting a glamour, a degree of self-shaping magic, etc) that he avoids using like the plague but are still there.
Magecraft: Want a talent for magic? Well here you go! Shrike has both the right kind of mind for it and the natural aptitude for channeling the magical power needed to, y'know, make spells work. Magecraft in his world ranges from rituals to enchantments to spoken spells, so if you can figure out the way to do it, well, there you have it. Shrike himself only has a very limited knowledge of spells so far, however, so he's not gonna be much help there, you'll have to figure it out yourself. :)
Geddoe
Lightning Rune: Basic, run of the mill lightning magic. With this rune affixed to your hand (or forehead, if you're really extra) you can cast a few spells that consist of variations of zapping your enemies with lightning. It's better for pinpoint attacks than aoe. The more powerful the spell, the harder it will be to cast repeatedly without resting, though!
True Lightning Rune: Like above, except this is pretty much straight up an elemental god of lightning hanging out in your right hand. It's pretty powerful, but it also has a will of its own: get careless, or be in a state where you're too exhausted/injured/whatever to resist, and the rune might just go berserk, blasting anything and everything nearby. Other things the rune might do include showing you visions of memories of previous bearers and/or possible futures to you, usually as dreams.
Swordsmanship: It might not be flashy as a magic lightning god, but Geddoe is good at what he does. Want to have some random sword skills of the learned via experience rather than fancy training variety? Look no further!
godslaying (+possession), halfbreeds, lightning magic, and more!
Sion
Magic: Let's start with the easy one. Sion's world has magic, and Sion is capable of using it! The way it works is, there's tiny motes of... something floating around the world. Normally these are invisible to humans, but someone who's undergone the right mental training can see them. And then manipulate them. Through a combination of physically interacting with and rearranging the motes into a pattern (usually with a hand/finger) and speaking an appropriate incantation, a spell can be cast! (To a non-magical observer this looks like you're creating a glowing diagram with your finger in midair, fun!) Spells include things like fire, a blast of water, temporarily deadening sound in an area, etc. Sion only knows Roland's spells, though in theory it'd be possible for him to learn others given the opportunity and sufficient time.
If you choose this option, feel free to ask me for info on incantations or such if you want! Or just have your character be puzzled by the floating glowing motes they now see everywhere, whatever works.
The Mad Hero: Now for the tricky one. This is a partial possession by/fusion with one of the gods of Sion's world, namely Aslude Roland aka the Mad/Fallen/Black Hero. It's basically a big package deal with multiple abilities and effects as follows!
Anti-existence black swords
These are literally the ability to slay gods, but the swords made of darkness can be summoned at will and used against pretty much anything. Things destroyed by the swords are effectively erased from existence and thus wiped from human memory, though your mileage may vary on that here in pokemon land. Note too that using this power comes with a price: every time you do, you'll become slightly less human and more, well, demigod-ish. Which will be elaborated on below.
Perception of the spiritual plane
... Look, you're basically now empowered by a god, of course you can perceive the dimension/plane/whatever you wanna call it that such beings normally exist on. That said there's two noteworthy drawbacks: first, that if you're not used to seeing/being in two places at once, it's hard to actually focus on more than one at a time. Second, and more importantly, while this lets you interact with other spiritual beings
so you can kill them with your swords of darknessthat also means that said other beings can now interact with and harm YOU, should they be so inclined. Think of it like humans being Normal types and gods being Ghost types so they're normally mutually immune; in exchange for being able to hit ghosts you now lose your normal immunity to them.Becoming a demigod/endless agony
This is a twofer. Since you're fused with a god and are (presumably) slowly assimilating his powers, you're not longer fully human yourself! Plus, you can accelerate this process by doing things like actively tapping into this power a lot. Push this process far enough along and you'll get nifty(?) side effects like glowing gold magical god blood! However, at ALL stages this comes with it incredible, unending agony. It feels like literally every individual cell in your body is somehow, impossibly, in pain. Nothing stops or dulls it, and it persists even if you're unconscious. The only way to block it is to block all the Mad Hero's power from you, rendering you normal human again.
Possession
Of course, the thing with trying to absorb a mad god's power? Said mad god tends to come with it. So enjoy having that presence in your head, constantly trying to take over and subsume your consciousness into his own. The Mad Hero basically wants to first break free of his prison (which... this place is NOT said prison, so that's a little moot right now) and second go finish killing all the gods. Humans are largely beneath his notice beyond how he can possibly use/get power out of them. The only being he MIGHT possibly care about (and that mostly from being reincarnated as a human countless times slowly changing him) is the Lonesome Demon, but since only half of the Demon is here there's not much he can do about that either. So really have fun with a very frustrated mad god wanting to rampage without anything to rampage at I guess.
Shrike
Fayblood: Shrike was sired by a fay, which means he's a human with a few... extra things tacked on. In his case this means he has somewhat funny-shaped ears, his finger- and toenails are narrower and thicker than a normal humans (think halfway toward being claws), and his whole body is covered with markings that vaguely resemble an animal's spotted coat. He's also unnaturally strong, capable of things like hauling a full-grown man wearing armor out of a flooding river, has a slight innate ability to kind of just sense magic, and technically has access to basic standard fay abilities (like casting a glamour, a degree of self-shaping magic, etc) that he avoids using like the plague but are still there.
Magecraft: Want a talent for magic? Well here you go! Shrike has both the right kind of mind for it and the natural aptitude for channeling the magical power needed to, y'know, make spells work. Magecraft in his world ranges from rituals to enchantments to spoken spells, so if you can figure out the way to do it, well, there you have it. Shrike himself only has a very limited knowledge of spells so far, however, so he's not gonna be much help there, you'll have to figure it out yourself. :)
Geddoe
Lightning Rune: Basic, run of the mill lightning magic. With this rune affixed to your hand (or forehead, if you're really extra) you can cast a few spells that consist of variations of zapping your enemies with lightning. It's better for pinpoint attacks than aoe. The more powerful the spell, the harder it will be to cast repeatedly without resting, though!
True Lightning Rune: Like above, except this is pretty much straight up an elemental god of lightning hanging out in your right hand. It's pretty powerful, but it also has a will of its own: get careless, or be in a state where you're too exhausted/injured/whatever to resist, and the rune might just go berserk, blasting anything and everything nearby. Other things the rune might do include showing you visions of memories of previous bearers and/or possible futures to you, usually as dreams.
Swordsmanship: It might not be flashy as a magic lightning god, but Geddoe is good at what he does. Want to have some random sword skills of the learned via experience rather than fancy training variety? Look no further!