Interesting detail, we didn’t see the Lich in Stakes.
Two-Bread Tom only mentioned Rainicorns and Vampires as the predators/enemies of the humans. Was the Lich stopped by another force before Marceline grew up into a vampire hunter?
The Lich was sealed away by Billy, but in Farmworld, Billy’s human counterpart is a regular aged human around his 60s. That means Billy in Ooo probably sealed the Lich merely decades ago.
Who sealed the Lich after the bomb?
Perhaps it was the only person who at the time could defeat him.
Simon Petrikov.