We just got an A5E version of The Archlich (no copyrights were harmed in the making of this stat block), and once again I can't help but express my pleasure at their design.
The spells are easy to read, but the spell slot system is preserved and actually serves a game purpose (Touch of Death gets more powerful depending on the spell slots you expend while doing it). There's even a special trait that says that the Archlich can cast
any Wizard spell as a ritual, which gives a DM so many opportunities!
I just don't understand how the Vecna block can come anywhere close to this in terms of interesting uses as well as ease of use.