Olroth

I've done about 20+ level 79+ expeditions(logbooks) but never saw Olroth. Am I doing something wrong or just bad luck?
Do you need to blow any special object/underground entrance to fight Olroth or it just appears as soon as you enter logbook area even before placing/detonating explosives?

Gotta blow up a special entrance to underground arena. (marked as such on map)

Found him once and died to a mechanic id yet to see. I'll totally get him next time-... next time.