The Lair of Tezakk Gnawbone is a lair hidden in the RvR zone of the Chaos Wastes. The entrance can be located north of Zimmeron's Hold, by the lake next to Chokethorn Bramble. When you locate the area you will get a Tome of Knowledge unlock.
In order to get to Tezakk Gnawbone you must first jump seven pieces of ruins to get to his entrance. Make sure you take your time carefully jumping each bit. Try having a running start each time. Once you reach the sixth ruin piece, do a high jump onto the high wall before you attempt the last ruin.