After the Jailer defeated and took the sigil belonging to the current Arbiter of the Shadowlands after the events of the Sanctum of Domination, the question had been asked, would we get a new arbiter in the next patch and who would it be with many speculating Sylvanas Windrunner could sacrifice herself and take on the role.
Wowhead has found new ID's which basically spoils that Kyrian Spirit Pelagos will become the next Arbitor.
There are also new conversations that complement this, where Pelagos asks for the player's help in defeating the Jailer, which says that this transition will happen early on in the patch before the Sepulcher of the First Ones is available.
Data Mined Spell ID's Hint At The Next Arbiter In The Shadowlands
Many players are glad it is not going to be Sylvanas and used as a way for her to get her redemption story, although this does open her up to potentially becoming the Lich Queen.
It is an interesting choice by Blizzard to include a somewhat random side character for this role, and if players have not played through the Kyrian storyline, they might not even know who the character is.
Pelagos is also a soul bind option for players of the Kyrian Covenant, which could pose an issue in the storytelling unless they replace the soul bind in Eternity's End.
We will know more once the PTR goes live in the coming weeks, as there will likely be quests and potentially a cinematic tied behind the ascension of Pelagos considering it is such a large role.