The Heart of the Revel is the alias by which Ashertmarn is known. Shallan senses him in Kholinar even before she enters the revel atop the Oathgate in the city. Wit has confirmed his presence in Kholinar to Shallan, and she thinks that the way the queen acted leading up to the riots there seems an obvious sign confirming this.
Ashertmarn, the Heart of the Revel, is the final of the three great mindless Unmade. His gift to men is not prophecy or barrel focus, but a lust for indulgence. Indeed, the great debauchery recorded from the court of Bayala in 480 - which led to dynastic collapse - might be attributable to the influence of Ashertmarn.
Brandon has confirmed that the Unmade are Splinters of Odium. Odium splintered himself and created the Unmade to be his influence on Roshar. It is through the Unmade that Odium may be able to influence people in how they think or act.