One of the biggest questions coming into the Shadowlands was, will we see Arthas Menethil, the previous Lich King before Bolvar and the main boss from Wrath of the Lich King, World of Warcraft's most popular expansion.
Many people didn't want to see the return of Arthas as his story was finished and to bring him back as an important character could harm his great story and legacy in the games dating back to Warcraft 3, but many thought there would be at least a cameo from the former Lich King considering we are in the Shadowlands, a realm he was closely connected to, and the fact he was featured in Uther's Cinematic.
So far there has been no sign of Arthas, but that may change in patch 9.2 Eternity's End with the first look at the new raid.
Sepulcher Of The First Ones Raid Contains Mechanics From The Lich King Encounter
This information has been data mined by Wowhead from the dungeon journal from the first PTR build, but it is not currently live and things are expected to change during the course of testing.
We know Anduin is the 8th boss, and will end week 1 as the final boss, and is expected to be as difficult as any final boss, despite Zovaal, the Jailer being the true boss of the Sepulcher of the First Ones.
Stage one has a mechanic called Kingsmourne hungers, which is a reference to Arthas' line "Frostmourne hungers" and features each of their weapons names.
Kingsmourne hungers is a mechanic which works like Harvest Soul from the Lich King encounter, banishing players for 35 seconds.
There is an Intermission phase in the encounter, which is called Remnant of a Fallen King, which brings back two more abilities from the Lich King encounter Soul Reaper and Remorseless Winter.
These abilities and the name of the phase are clearly a throwback to the final fight against Arthas as the Lich King.
The final throwback is in the Jailer fight, where Zovaal uses Defile, a big mechanic from the Lich King encounter in Icecrown Citadel.
Players might need to wait a while before they get to see these mechanics in action as there is a high chance that Blizzard may not test the final bosses of the Sepulcher of the First Ones.