Boss killed being dead, quest item lost

Hello,

When I killed the boss "King in the Mist," I died just before, and he died immediately after.
As a result, I couldn't obtain the spirit because the items disappeared...

How can I retry the boss fight and get the quest item that grants the spirit?

Additional details:

I'm in Cruel mode.
The tree was bugged, and I couldn't open it either.
The boss icon is grayed out on the map.
I tried killing him again but still didn't get the item (although the tree was no longer bugged).
3 419 784 261: Reference after using /bug.

Thank you!

UP ?