Karthus rework concept
Passive: reaper: karthus has a pet reaper who roams the map looking to reap low health champion. The Reaper will move slowly towards enemy champions who are below 50% health no matter where they are. The reaper gains speed when moving towards enemies below 25% health and even more when below 10%. The reaper cannot be killed although it can be slowed or stunned.
Q: karthus defiles the ground in a location. It leaves a pool of death on the ground dealing magic damage. Defile grows in size and extends the duration If an enemy champion is standing in it. (Cooldown 12)
W: karthus leeches health from an enemy dealing magic damage (channel while moving)
E: karthus fears enemies in a cone in front of him for 1 second. Feared enemies are tethered together and take 10/20/30/40/50% damage other tethered enemies take for 3 seconds.
R: reap. Kathus channels for a second then appears behind each enemy champion dealing 10% true damage max health damage to each of them. (Think karthus current ult combined with Fiora old ult)