I'm looking to build a Roy Mustang (from FMA) style character, whose primary method attack is a fiery explosion. While there are many Fire-based spells and cantrips I can use, I'm focused on the interaction of the Kiss of Mephistopheles eldritch invocation (see Unearthed Arcana: Warlock & Wizard) and the eldritch blast cantrip. For this character, it would make more sense for him to launch a flame, rather than a beam of force, that explodes into a fireball.

Is there a RAW way to adjust the damage type of eldritch blast to fire? I would be happy with a partial damage split, as long as there is fire damage being dealt.

Conditions:

• He has to be Warlock 5, but multi-classing is allowed




• We are allowed to use the official material, plus UA material (after running it by the GM)




• Magic items will work, but they will need to be somewhat easy to get ahold of.

Note: The GM would likely be fine with allowing me to change the damage type, but I'd prefer using existing content that allows this instead. There have been cases of "You changed a rule for them, so you should change a rule for me" and I want to avoid this.

There currently is no method for this

While the Unearthed Arcana Lore Wizard created a method to change spell damage types, it only included spells that cost a spell slot, so cantrips like Eldritch Blast were not included.

In terms of changing a damage type for any spell, that was your only option.

However, even if you did utilize that, you really have to be careful in multiclassing Unearthed Arcana classes - those weren't tuned for multiclassing and combining a UA class with a UA invocation is treading into risky waters.

DM's choice to change it

A DM could provide EB as fire instead of force. Ultimately, it's not a big deal, but it may become a decision you regret because there are many monsters with fire resistance/immunity and very few force resistant/immune.

Changing for flavor

While Mustang does use actual fire here, you could just 'flavor' the look of your eldritch blast to be a fire-looking effect - but that it does just deal force damage. If you don't change the actual mechanics (damage type), then you can change your narration for it - and maybe just for when you use the invocation.

A build challenge

Instead of going with your current build, you could do a Sorcerer build that has fire bolt and the Metamagic Quickened Spell so that you can cast the cantrip fire bolt and follow it with a quickened fireball bonus action.

If you want to minimize risk here with fire being often resisted, you could also take the Elemental Adept feat (PHB, p. 166) and bypass fire resistance.

• Spells you cast ignore resistance to fire damage. In addition, when you roll damage for a spell you cast that deals fire damage, you can treat any 1 on a damage die as a 2.




• You can select this feat multiple times. Each time you do so, you must choose a different damage type.

There isn't really a way to do that officially, but I wouldn't worry too much about having the DM do it by fiat -- changing Force damage into Fire damage is actually a serious downgrade, as fire is the second most commonly reduced¹ damage type (after poison), while almost nothing reduces pure force.

If the DM gets assailed by requests to alter other rules, there's an easy response of "Okay, that's fine, as long as you're intentionally weakening yourself." (But to be honest, that is really not your problem; let the DM refuse your request if he feels it sets a problematic precedent, and if he doesn't feel it's a problem, you don't need to protect him.)

¹ When I say "reduced", I'm collectively referring to both immunity and resistance.

There will likely never be a RAW way to do this. In your own game, though, it shouldn't be unbalanced, with a few caveats.

Currently, there is no way to change the element of Eldritch Blast, and that's important, as there is also no way to specifically buff force spells. Eldritch Blast can be very easily made into the most powerful cantrip in the game by a wide margin, making Eldritch Blast optimization a significant area of endeavor. Being able to cast it as fire rather than force would unlock a number of things.

• Celestial warlocks at 6th level can add Cha mod to damage on one damage roll per cast of a fire spell.




• Draconic Bloodline sorcerers at 6th level can add Cha mod to damage on one damage roll per cast of their favored element (and also get temporary resistance to that element at the same time). Fire is a permissable element.




• Tieflings can take the Flames of Phlegethos half-feat (Int/Cha) that allows rerolls of 1s on fire damage from spells, and also provides a temporary aura that deals small amounts of fire damage to attackers.




• Technically, Elemental Adept (available for fire, but not for force) lets you turn rolled 1s on damage dice into 2s, but that's admittedly pretty weak for a d10 damage die.

Now, admittedly, these are all pretty low-intensity. None of them is going to give you overwhelming power, and the costs are nontrivial. Still, Eldritch Blast is already pushing the outer bound of what's possible for at-will damage for primary spellcasters. It appears that the fact that you cannot change the element is by design.

More importantly, it may well not apply to your character specifically. If you want Kiss of Mephistopheles, then you're not a Celestial warlock. You may well not want to spend 6 levels learning how to be a dragon sorcerer. If you're not a tiefling, then no Flames of Phlethegos... and Elemental Adept probably isn't worth taking regardless. If you're willing to agree to not use the various boosters for fire spells, it should be no problem... but there is not now and will likely never be a way to do it. That's the sort of 4e optimization that the 5e authors have systematically suppressed.