Rules clearification before I have to deal with this as DM (New Undead Warlock)

The new undead patreon has a rule that has a vague loophole that needs a clarification on. 

Arcane Necrosis. Necrotic damage from your attacks, Warlock spells, and Warlock features ignores Resistance to Necrotic damage. Once per turn when you cast a spell that deals damage, you can change that spell’s damage type to Necrotic.

at level 6: Once per turn. issue. Because some spells target multiple targets with multiple attack rolls. Do all the rolls get turned into Necro damage, or just one of them. 

example: Eldrich Blast .... which your warlock will be useing.  At  level 5 it becomes two so the Warlock can they make both Necrotic? 

Quote from gothicshark >>

The new undead patreon has a rule that has a vague loophole that needs a clarification on. 

Arcane Necrosis. Necrotic damage from your attacks, Warlock spells, and Warlock features ignores Resistance to Necrotic damage. Once per turn when you cast a spell that deals damage, you can change that spell’s damage type to Necrotic.

at level 6: Once per turn. issue. Because some spells target multiple targets with multiple attack rolls. Do all the rolls get turned into Necro damage, or just one of them. 

 

example: Eldrich Blast .... which your warlock will be useing.  At  level 5 it becomes two so the Warlock can they make both Necrotic? 

Based on what you've quoted here both beams from Eldritch Blast will be changed to Necrotic damage if you choose to use this feature.  In fact, you cannot change only one of the two beams.  

The decision to use the feature or not is made "when you cast a spell that deals damage" and the result is that "you can change that spell's damage type".  The spell in question is Eldritch Blast, so the damage type of that casting of Eldritch Blast can be changed.  If your Eldritch Blast happens to generate two beams, then both of those beams are affected.

Just to expand on what up2ng said, when they want to limit this kind of thing to a single damage roll, they always say that explicitly.

A good example is found in the Reanimator Artificer subclass from the same book, which has this "Arcane Conduit" feature that says (emphasis mine):

Once per turn, when you cast an Artificer spell from the Evocation or Necromancy school and deal damage while your companion is within 120 feet of you, you can add your Intelligence modifier to one damage roll of that spell.

Since Arcane Necrosis does not have language like this, it does not have this limitation.