Mods involved -
MineColonies
Chance Cubes
Yours.

I broke a chance cube, The effect is that, days later, a phantom anvil appears, hits you, and kills you dead. My grave apparently broke under the attack, and was picked up by my builder from Minecolonies. I found it in their inventory. It is now in mine. How do I open it and get my things back, if that is possible?

I suspect dying in an area they were upgrading is a factor.