Not sure if it is really a bug or the fact that I might have done something wrong, but sometimes when I died as Karthus I was unable to use my Ultimate while dead despite not using any other abilities and the skill was not on cooldown.
I'm not sure but I think I usually had defile on before dying.
HopFrogger1 votesIf you try to ult too late in your passive, you will no longer be able to use it. Sounds like you might have waited too long to ult?
reapshot1 votesOh I'm sorry I didn't clarify. When this happened to me I was spamming ult right after dying because a lot of the enemy team was nearly dead every time. Sometimes I would Q 1 or 2 times (right after each other spam Q) before enemies ran out of my range so I tried to ult afterwards since no one was in range and couldn't ult.