Passive:
Galdelonian gains CDR equal to .1 of his ability power.
Thus 10% would be from 100 ability power. Considering his abilities would have high base cooldowns. This would mean you could build anything

Q: Galdelonian reveals a small portion of the map (Kogmaw's Ultimate radius.) for 2 seconds. This ability would deal 10/20/30/40/50 damage + .3 of your ability power. CD 23/20/18/15/13 with a mana cost of 45
W: Galdelonian (at the range of Malphite' Q.) invades target enemies mind. Making it harder for them to focus. This ability lowers their range of sight. The range is lowered based on the level of the ability. This ability would deal 5/10/20/30/35 damage with a .1 ability power ratio. 30/27/24/20/18 with a mana cost of 60
E: Passive: Galdelonian gains health regen and mana regen with every spell cast. 5/10/20/30. Active: Galdelonian channels this spell for 2 seconds. After channeling Galdelonian loses magic resistance for 10s. Any spell damage dealt to Galdelonian during this time will be converted into health and mana regen. Equal to half of the damage dealt. This effect lasts 10s. During which the passive of his ability becomes inactive. CD would be 40/38/35/30/28 as this move would be phenominally dangerous and life saving.
R: Galdelonian takes contol of target enemy champion (Kogmaw's Ultimate radius.) for 2 seconds. During this time Galdelonian may use their abilities. This ability doesn't work on minions or neutral creeps. While controlling an enemy unit Galdelonian is immune to crowd control but whenever damage drops him or his target below 50% health the spell is broken. While controlling a target Galdelonian uses his AP for spells used by those champions. This ability would cost 100/150/200 mana. The cooldown of this ability would be 120s, 90s, 75s.

The ultimate can be cleansed unless it could be a suppression.

The balance of this character is that he cannot deal damage. He is mainly support and it is hard for him to farm minions. Overall taking control of enemy champions would be nice but since he cannot do anything while using his ultimate it would be equally as dangerous. The utility and support of this champion would be great. It would be possible to build him as a tank as he doesn't need much to keep going. As his mana costs would be generally low. He could also be built AP so his Q and W would be particularly dangerous whenever you're near neutral creeps that he wants to check.

Tips:
Against:
Galdelonian is weak when it's 2 v 1 whenever his ultimate is on cooldown. Don't focus him too much because with his low health he'll quickly drop.
Take advantage of your teammate stopping Galdelonian from ruining your efforts in killing others with spells. While his ultimate is being used judge weather it's for the better or worse of your party. For instance if Galdelonian took control of Garren and all of his abilities are on cooldown and he's in tank gear you can just let his ultimate last it's full duration because unless Galdelonian or his target take damage that drops them below half health he is stuck in that body and that gives you a free shot at the rest of his team.
Don't fear Galdelonian's spells for their damage is quite weak. Just make sure not to stand in the same spot for too long or he may get free pokes with his Q.
Strategy:
Galdelonian's Q helps him line up his ultimate's use. Don't be afraid during team fights to hide off at the side for an escaping Mundo using his ultimate so you can pick them out and throw them back into the fray. Burning agony cleaver and masochism and all.
It is a good idea to take control of lower health targets with high resistances that need to sacrifice their health to use abilities. This would allow for a quick win in team fights and also ensure if your team does lose you can quickly exit the posessed target.
Galdelonian's W will make it harder for enemies to cast spells in combat. This is especially useful against long range spell casters like Ryze who require a target for their spells.