Looking for suggestions for creating a warlock spell

[TieflingAmon]

Hello I am a long time forum lurker and first time poster looking for some suggestions.

I am currently playing a evil tiefling warlock (pact of the chain) in a custom d&d 5th campaign and I am level 12.

I have been informed that if I should want to invite Mephistophles favor that I am to either uncover or develop a new weapon that will impress him. I instantly thought of developing my own new spell.

I was looking to develop a 5th level spell with a hellfire theme. My first idea was just a unresistable hellfire typed damage spell that did bonus damage to celestials. I then though up a spell that did damage and if it dropped a celestial's hitpoints below a certain threshold then they would be banished back to there plane for a year. Both of these options i am not to impressed by. I was hoping to gather some interesting ideas from the community.

 

[MessiahMushroom]

What sub-type of warlock are you? Have you taken Pact of the Blade, Tome, or Chain? I feel like your custom spell should play with whatever strengths you personally have.

 

[TieflingAmon]

I am pact of the chain. It's a very rpg heavy campaign and are GM is quite liberal with house rules. As an example throughout my past exploits in game asmodeus blessed me with the ability to polymorph into Devils instead of beasts.

 

[MessiahMushroom]

Considering you'll probably take Chains of Carceri as an invocation at level 15, you may want to steer clear of creating a spell which interacts with celestials so as not to step too much on its toes (it's such a cool invocation).

I think it'd be interesting to somehow utlize your familiar in the spell creation process. Depending on what you've chosen as your familiar, you could temporarily transform it into a more powerful version of itself, merge with it to become a weapon yourself, unleash a devastating ability it's been bottling up, or even sacrifice it to deal damage to your foes or buff yourself for a time.

If any of these ideas pique your interest, we can look more in this vein or brainstorm some more.