Obsidian Hall

Also known as the Hall of the Duels, the Obsidian Hall is a huge chamber shaped like an eight-sided prism, where Magisters of the Colleges of Magic compete for the station of Supreme Patriarch. The thick walls, the ceiling and the floor are made of gloss smooth obsidian. The black stone is the bane of magic, totally refractory and inactive. Wizards can feel their power dimmed by the presence of so much while all around them.

Under the symbol of their own Wind of Magic, the Masters of the other Orders stand in alcoves carved in the eight walls, protected by powerful enchanted barriers. All of this is necessary to contain the powers that both defendant and challenger will unleash during the match. The objective of the duel, however, is to reach the centre of the Hall, where an altar stands shaped in the guise of the Great Wheel of Magic. The Staff of Volans--the Staff of the Patriarchs--levitates above this hub.

The first contender to lay their hand on the Staff will immediately have their powers amplified by the ancient artefact. At which point the other wizard normally surrenders, if they know what's good for them. Anything is allowed to stop the opponent from approaching the altar, and in the past there have been fatalities among the contestants.

Source
pg 54
 * Warhammer Armies: Empire (6th Edition)