As with most other bosses in Perdition Rock, Rwek Khawl Mawl will not always spawn. If you don't find him, you can re-zone and try again.
If Rwek Khawl Mawl Echoes another skill before dying his Echo skill will be replaced with the other skill. If you want to capture Echo but it does not show up in the capture skill list, wait for 30 seconds and use Signet of Capture again.
The boss' name seems to be words similar to "Wreck, Claw, Maul."
Like all Hydra bosses he has three names, one for each head.