Rivalen Tanthul

History
Rivalen Tanthul is the first of the twelve Princes of Shade. Rivalen is an accomplished wizard but also The Nightseer, a high priest of Shar, the goddess of loss and night. This makes him what the Netheril call a Theurge, blending the magic of the Art and the Power. If High Prince Telamont is the temporal ruler of the Shadovars, Prince Rivalen is their spiritual leader. From the massive Hall of Shadow located near the royal palace, the prince administers Shar's clergy and pronounces her will to the faithful. The prince's divination enabled his father to finally guide the city back to Faerun, where Shar hopes the Shadovars tip the balance of power in her favor in the long struggle against her hated sister, Selune.

Rivalen is also Hadrhune's greatest rival. He resents the favor that his father shows to Hadrhune, and is secretly working to undermine him. He knows that he must exercise great caution, because Hadrhune's enemies have an unpleasant habit of disappearing.

Prince Rivalen's personal sigil is the symbol of Shar; below it is a pair of curving dark purple ram's horns, outlined in black on a dark gray field

Rivalen in The Twilight War trilogy
Rivalen orchestrated, with the help of his brother Brennus, the takeover of the land of Sembia and its reduction to a vassal state of the City of Shade

Rivalen Tanthul is now a demigod/exarch, having absorbed a shard of Mask's power after helping Erevis Cale and Drasek Riven kill Kesson Rel.