Gains charges upon moving or casting. At 100 charges, the next spell hit expends all charges to deal 100 (+0.15 ability power) bonus magic damage to up to 4 targets on hit. Applies spell Effects (e.g., Spell Vamp, Rylai's, etc).
How does this behave with Karthus ultimate?
Best Answer
From a Reddit post explaining the behavior between Luden's Echo and Karthus Ult:
This is the TL;DR, did not want to post the entire article here. If you want to give it a read, you can check it out here: http://www.reddit.com/r/leagueoflegends/comments/2yskwz/ludens_echo_karthus_interaction/