Hello andycarp42 and welcome to the forum
This is a map of the 2nd level of the catacombs to find slasher :
http://kiya1.via.t-online.de/images/Div_ale2.jpgBut i think it is bad for you. The biggest orc is not the leader and i don't know if you can end this quest because you killed the orc chief.
Try to go and get slasher, if it does't work, restart the game you are just at the beginning.
Barta