Dexterity save for Leo to cast Bless prior to people being hit by the gas? If so, it would be going on those closest to the door, himself, Zinna, and (I think) Corren.
In a similar vein, how far did that gas spread? I believe both Corren and Mradoric described themselves as being a fair ways down the hall.
Corren would only note that the door is made of heavy/sturdy oak with brass hardware/fittings. He wouldn't have found anything to suggest that the door is trapped, only that it is locked. The locking mechanism is old but despite its age the mechanism remains intact (It would require a successful Dexterity/sleight of hand check using thieves tools DC 11)
Of those in the corridor, Mradoric may be out of range of the gas if he was positioned closer to the intersection; if he was then disregard the save/damage for Mradoric. Corren was accurate in his description in the Main Thread about his position near the other door and would have been exposed.
That makes sense; some spells like ray of sickness wouldn't make sense to have work against undead because it causes poison damage which the skeleton is immune to but it does make sense that necrotic energy could take a number of forms, like breaking down or causing bones to become brittle, so we will include the damage from hex.