A Summit of Mages is one of the main-quests that must be completed in the third chapter of the Witcher 2, in order to beat the game.
It is a short quest, but the characters that appear during the cutscenes can vary, based on the decisions you have made so far in the game.
A Summit of Mages is activated after you complete Where is Triss Merigold or The Spellbreaker quests.
If you already completed Where is Triss Merigold?, then you will meet Triss near the amphitheater in Loc Muinne.
If you played The Spellbreaker, instead of Where is Triss Merigold?, then you will meet Iorveth, assuming that you sided with him at the end of the first chapter.
If not, Roche will be your companion during this quest.
If you decide to play A Summit of Mages, you should know that you will also start the final part of the game, which means that you should make the necessary preparations, and you should complete all side-quests you have activated so far.
“Alone on Sorcerers summit, the witcher would feel like a raftsman on a declamation contest.
Luckily, Triss Merigold awaited him at the amphitheaters entrance.”
When to start
After Where is Triss Merigold?/ The Spellbreaker and after you complete all side-quests
When you are ready to begin, talk to Triss/Iorveth/Roche near the amphitheater, and kill the Redanian Knights guarding the door.
Note: At this point you should save the game, and then accept to enter the amphitheater.
Inside you will witness a long cutscene which culminates with Sile being accused by Triss (assuming that she is you companion), for treason.
The conclave decides to arrest Sile, but Saskia (in her dragon form) helps Sile and takes her to a tower.
Then the mind-controlled dragon returns to the amphitheater and attacks you and your party. A Summit of Mages ends and Enter the Dragon quest begins.