ToME: the Tales of Maj'Eyal

Im sneaking (lvl5) in the Valley of the Moon.
Only a few stray monsters came down quickly killed ofc.
After a while I feel something is wrong and I move up towards the entrance.
Up there, in the top right corner are heaps of monsters gathering.
Actually the whole area is packed of them.
I do hope they will not make an attack at the same time, but so far nothing happens when I just stand still pressing '5'.

I just had this happen to me, and I wasn't sneaking. Playing a summoner standing on the island. I have telepathy all so I can see the demons in the top right corner but no one moving. A duathedlen keeps casting rune shield and it runs out and he casts it again. Got the achievement when the time ran out. They stopped moving with around 150 left on the timer.