So this is a champion concept based on a Warframe. If you don't know what Warframe is, it's basically a combi of a 3d person shooter and LoL.
Q: Soul Portal
Limbo opens a soul portal. This soul portal drains the soul of nearby enemies (in a cone in front of it). The portal is opened on a wall and can be interacted with.
If a soul portal kills an enemy, the enemy will be send to the limbo-rift as a soul. Limbo can pick up the souls in the limbo-rift to gain health.
Limbo transfers himself into the limbo-rift for up to 6 seconds. In the limbo-rift, he takes less damage but can't damage enemies unless they are in the rift themselves as well.
Limbo is also 40 to 60% faster in the limbo-rift with absolutely everything.
Limbo banishes enemies to the limbo-rift. He does this by pulling them into the Soul Portals. If a minion is pulled into the Soul Portal by Banish, they will instantly die.
If an enemy champion is pulled into the limbo-rift, they will be stuck there for 6 seconds, taking additional damage and dealing less damage to non-effected enemies.
An enemy can try to leave the limbo-rift by dashing, jumping or blinking into a Soul Portal, but that will deal damage to the enemy.
R: Awakend Soul
Limbo draws energy from the living forces near him, dealing a small ammount of damage to them. Then, he gets a buff called Spirit Walker. While this buff is active, Limbo can use Transfer without a cooldown. While in the limbo-rift, Limbo will deal bonus damage instead of none and will take no damage. The buff's duration is effected by the ammount of enemies hit, adding 3 seconds for every enemy champion hit and 1/5th of a second for every minion or jungle monster hit.
Additionally, enemies inside the limbo-rift will take continuous damage.
The idea with Limbo is to transfer as many enemies to the limbo-rift as possible, and fight in either a small area to make his portals deal as much AoE damage as possible, or fight in a minion wave so he can kill the minions with his portal to pick up their souls with Transfer if he gets badly hurt.