The Bladed Path quest in Season of the Witch concludes this week, but it opens up the fourth arena inside the Altars of Summoning activity. It's a three-player matchmade seasonal arena where offerings — which determine the difficulty level of the encounter — summon bosses and crystal mechanics. The space is divided into four parts, and in the latest Altar of the Sword area, players can fight a new Knight boss called Arok'tha, Sworn to Xivu Arath.
The Destiny 2 community on Reddit reports that players can summon the new boss with any tier offering, but only in the Altar of the Sword area. Although players unseal the Altar of the Sword through Mission: Sunder in The Bladed Path quest, those who haven't done the mission can still access the area in the Altars of Summoning. Nonetheless, RNG determines whether players face Arok'tha or not during their run.

The Arok'tha encounter is similar to the Leviathan-Eater, Bane of the Ammonites boss fight in Mission: Sunder. Once Arok'tha walks through the giant Hive portal, players can damage the boss and complete the Solar Crystal objectives to remove its shield.
The in-game text labels the Arok'tha encounter as "War Brood," but the Acolyte's Offerings seasonal Triumph lists it as Xivu's Brood. Acolyte's Offerings asks players to complete all encounters in Altars of Summoning, and those looking to cross off this Triumph can do so by defeating the new boss. New bosses have entered Altars of Summoning throughout the weeks, but it seems like Xivu's Brood is the last one.
Even though the Season of the Witch quest ended, but there's more to uncover like The Imbaru Engine puzzles, secret chests in Savathun's Spire, and elemental attunement Opaque Cards.