Underworld Hallway

Cumulative upkeep - Destroy target creature (At
the beginning of your upkeep, put an age counter on
this permanent, then sacrifice it unless you pay its
upkeep cost for each age counter on it.)
For each two creatures destroyed by the
cumulative upkeep cost, add an age counter
on this permanent.
Whenever you sacrifice this permanent, it deals
damage equal to its age counters to you.