Complex sentence: Although Homunculus and TSTF were quietly working as "sleepers" at the time, they have taken centre stage since the involuntary retirement of Dilip Olaf and asdfg.
Split into simpler sentences: Although Homunculus and TSTF were around at the time, they were "sleepers" well under the radar. They have taken centre stage since the involuntary retirement of Dilip Olaf and asdfg.