The Summoner is on a platform with fire throwing ghoul lords. It's a really nasty situation, you will clearly need some distance attacks. Finish him off and read the tome, which will open up the gate to the Canyon of the Magi, it will also tell you which is the correct Tal Rasha tomb. Now you can head into the correct one straight away or you hit all the other tombs for more exp and loot. If you forget which tomb to look for use the quest log.
Head through the tomb until you get to the orifice, once you are there get close to the orifice with the staff equipped and touch the orifice, a nice magical displace will take place and the last level of the tomb will open. Head through the opening prepared for a battle. Kill the worm guy there and head inside.