There seems to be an issue with the Blockheads mod and the tile entity of the sarcophagus from the Atum.

Atum version: Atum-1.15.2-2.1.1.jar
Minecraft Version: 1.15.2
Forge Version: 1.15.2 - 31.2.31

Crash Report:
https://gist.github.com/TheWitherlord/c8add0e5464c237a921d613c31f696e7

Latest Log: https://gist.github.com/TheWitherlord/bda2ab14d5deee2b040500f03f048fe0

Hope this is enough info to understand the problem?

Duplicate of #189 and #190.

As sscards said ^
You can grab a Jenkins build with a fix here:
https://jenkins.girafi.dk/job/Team%20Metallurgy%20Mods/job/Atum%202/job/Atum%202%201.15.2%20Jenkins/
A new build with probably be out on CurseForge later today