Tomb of the shadow king appears to have finally been fixed after all these years.
Yesterday I ran it like always while leveling a TR. To my surprise, the mummy in the end (beginning?) was taking zeroes for damage once he became active, like he's supposed to. Up to about two weeks ago, you could just kill him right after he appears, but now you have to use the quest mechanic with the cage & the light to finish.
The funny thing is, I've used the cage exactly twice now. Once about three years ago when I trusted the quest mechanics to not be broken (I think they were), the second time yesterday, because the quest finally worked as intended.
Thanks devs, you made my day!