Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding Scripting Osiris/API/ItemIsInInventory

Got it working for people who are interested:

IF
ObjectFlagSet("CheckItemRune",(CHARACTERGUID)_Player,_Instance)
AND
ItemIsInInventory((ITEMGUID)ITEMGUID_LOOT_AtaraxianArtifacts_A_000_2a36a9b7-c5a4-47ea-955d-bcbfa659efa5,1)
THEN
ObjectSetFlag(_Player, "HasItemRune");