Monster packs spawn randomly, there is no way to know
but there are 5 type of monsters in the pit:
Devilkin,
Dark Stalker,
Dark Archer,
Bone Warrior
Devilkin Shaman
and elites may spawn in all or a couple in those packs
Each area has a range of how many champ/unique packs can spawn in total.
There was such a list on amazonbasin, but the site's down for some reason.