Unnatural is a form of Fragile. Anything with it (summoned, conjured, or a magical or weird-science “construct”)automatically fails the HT roll to stay alive if reduced to -HP or below, as that much damage severs the ties with the force that animates you.
Advantage stacking[]
Kromm in Aug 2005 http://forums.sjgames.com/showthread.php?p=104933 stacked with Supernatural Durability
PK in Sep 2005 http://www.sjgames.com/pyramid/sample.html?id=5583 stacked with UK3
quotes[]
2018 Kromm http://forums.sjgames.com/showpost.php?p=2217047&postcount=13 agrees with Fragile (Unnatural, -HP) and Fragile (Unnatural, 0 HP)