During each opponent's upkeep, Urabrask's second ability triggers, causing them to exile the top card of their library the next time they would draw a card this turn.
During the opponent's draw step, they exile the top card of their library.
Due to Drannith Magistrate, the opponent is unable to cast the card exiled by Urabrask.
This lock completely neutralizes your opponents' draw steps by weaponizing Urabrask, Heretic Praetor's replacement effect. Instead of drawing normally, your opponents are forced to exile the top card of their library and attempt to play it from exile. However, because Drannith Magistrate forbids opponents from casting spells from anywhere other than their hands, that exiled card becomes unplayable garbage trapped in limbo.