When your mode is open in Engine
1. Open story editor
2. In File menu select "Generate Definitions"
3. Once done - close story editor
4. Open story editor again
5. Click "New script" and give it some name
6. In KB section of script input code I gave you:
IF
CharacterStatusRemoved((CHARACTERGUID)BOSS_GUID_HERE, "INVISIBLE",_)
THEN
ApplyStatus(BOSS_GUID_HERE, "INVISIBLE",-1.0,1);
But in place of "BOSS_GUID_HERE" place GUID of the something that you want to keep invisible
7. In file choose Build and Reload
8. Make sure that it is initially invisible
9. Run your level and test
Also check this:
https://docs.larian.game/Osiris_Overview