Soulforge started rotating troops from the 2nd week of January.
There are 27 mythics and having 4 per week gives about 7 weeks to rotate the pool. So worst case if a troop is at the start of a pool and end of the next is about 14 weeks.
Infernus was last seen 8 weeks ago. So it could appear any week now, and is certain to turn up in 6 weeks, assuming troop rotations do not change.