You begin prepping for ambush as you get up into the cave you find a large set of rocks you hand off of almost similar to some of those night-lords. As you wait slowly you see several human figure enter the cave many of them holding sticks others with primitive looking bows and axes. As they come in they slowly search the cave soon crossing your sacrifice site.

As they look at the site several of them begin talking in there own language you can't tell much what they are saying. As one of them approach the site they smell the blood and the rotting tau corpse thats are in there place. some of them go to touch it but back away as another in the back begins almost looking frightened for a moment with his spear and gestures back to the mouth of the cave.

You seem to be in a good spot for a ambush or possibly a place to make your self known or even strike terror into there hearts.
Options
A Spook them
B ambush and begin cutting down
C Show yourself and try to talk to them or even convert them, even though there is a language barrier
D Player choice

D. Present yourself as a deity and messiah from the dark gods (possibly spooking them) then demand their obedience and worship of the chaos gods as well as their allegiance to you as their warlord.

KnightReborn´s D if we have an idea to overcome the language barrier.
Otherwise (my choice until I edit this out): D: Wait until they leave and follow them.