The Thanos Quest Comic
Thanos peers into the Infinity Well. Mistress Death asks him why he has not begun the work she has assigned him: killing half the population of the universe and helping her balance her power.
He tells her it would take him centuries to accomplish this goal with his current power. He suggests she allow him to find the source of ultimate power, The Infinity Gems. She accepts once she learns that Thanos will get the first one from her enemy, the In-Betweener. The In-Betweener had cause her trouble in the past and she will be happy to get rid of him.