Find the room called The Keyhole in the Curse of Xazra and enter keyhole it to obtain the mark.
WARNING: This will place you inside New Kolvir, at the The New Kolvir blademaster.
For traversing the hellscape that is the interior of Xazra's chest and emerging from within, the Powers granted thee a mark! You have been granted The Mark of Rebirth! You gain 250 practices!