My character serves the life-giving sect of a duotheistic religion. However, I was talking with my DM about multiclassing and he suggested I could multi class into a fiend warlock with my patron as the other god, an underworld deity. He has come up with a possible encounter that could spur the change. What do you think I should do? 

It's a rough combination, since Cleric keys off of WIS and Warlock keys off of CHA. It's do-able... I think you'd have to pick which is your "Main" class and build your character around that, and then focus any spells learned from the other class on buffs and utility spells that don't call for attack rolls or saving throws.

If you want to go for something that would work a little better functionally, if you could instead play a Divine Soul Sorcerer, which gives you full access to the Cleric spell list, you would only have to focus on Charisma. Warlock is great for multiclassing, and pairs really well with Sorcerer. 

Is this something you want to do or something the DM wants to do? Working together is great, but I know sometimes another person's enthusiasm can kind of take over and push you in a direction you might not want to go. Your character is the story you want to tell, the DM can tell the story they want to tell with the rest of the campaign world. But if this is a concept you are interested in, cool.

Mechanically, dual-stat caster mixes are tough. I agree with what trans says above if you want to go this route. I want to stress that warlocks are not the only class that can make deals with powerful entities - all the pact stuff is just descriptive fluff that you could apply to any class if you wanted to.

I'll take the opposite view - I'm a big fan of Cleric/Warlock... and remember, you don't necessarily have to have a 'positive' relationship with your Patron.

There are a few great threads about Cleric / Warlock multiclassing in the Warlock sub forum. You should check those out.

And don't worry about your stats... as long as your Cha is 13 or higher (required for multiclassing into 'lock) you'll be fine. Fully optimized? Not necessarily... but you'll do fine, and the RP (and versatility) is great. Look hard at the Pacts and the Invocations... for my money, that's where Warlock really shines, and allows for customization. Don't just try to add Eldritch Blast to your Healer... think about what you're going to bring to the table, and why your character wants / needs that kind of power from your patron.

Agree that Divine Soul Sorcerer can do a lot of the same spell selection and healing focus as a Cleric. But also, just being a Celestial Warlock can pack that multiclass into a single class as well, giving you your usual Warlock invocations, spell selection, and Pacts but also some limited cleric spells, a healing feature, and some holy-type features at higher levels. A class that can both cast Cure Wounds and Arms of Hadar should be able to do a good amount of work telling your narrative story of duality, without having to mechanically multiclass.

However, it sounds like you may already be a Life Cleric, and more interested in how to add in a Fiendish element to that than vice versa. From that, I take it that you probably have good wisdom and con, maybe a decent str to wear heavy armor, and probably low-decent charisma for the warlock multiclass. You probably aren't focused on Strength- or Dexterity-based melee attacks currently, you're probably using Wisdom-based spells and cantrips. Good news is:

• you don't need high charisma, unless you plan on attacking or using save-based spells from your warlock multiclass. The only fiend features that specifically need charisma are your THP bonus from killing enemies (not consistent enough to worry about), and many of the bonus spells that are added to the warlock list for you (which you aren't compelled to take). Instead, you could just learn Warlock spells that don't attack/require a save, and experience no real penalty from low charisma at all, while benefiting from other Warlock features, invocations, and pact boons that don't check charisma.
• your Life Cleric bonuses to healing spells (both Disciple of Life and Blessed Healer ) don't require Cleric spells, they're happy to work with any spell that restores hit points. However... I can't think of many on the Warlock list that restore HP, other than Vampiric Touch, and then we're back into the problem of avoiding Charisma above...

Really... Fiend probably isn't the Warlock subclass you want if you're looking for one to provide you low-Charisma benefits. Just as the narrative need not bully you into a Warlock multiclass, it also shouldn't bully you into the Fiend pact, you can tell that same dual divinity story even if your pact is mechanically Genie, or Archfey, or whatever. Fiend isn't bad, but I'm a big fan of Genie, since it gives you several cool features, and has a bonus that scales on character level rather than warlock level or charisma, which will better help a multiclass dabbler. Celestial is pretty charisma-dependent and probably overlaps rather than enhancing your cleric features and spells... but Undying would be another decent low-cha splash class. 

Quote from scatterbraind >>

Is this something you want to do or something the DM wants to do? Working together is great, but I know sometimes another person's enthusiasm can kind of take over and push you in a direction you might not want to go. Your character is the story you want to tell, the DM can tell the story they want to tell with the rest of the campaign world. But if this is a concept you are interested in, cool.

Mechanically, dual-stat caster mixes are tough. I agree with what trans says above if you want to go this route. I want to stress that warlocks are not the only class that can make deals with powerful entities - all the pact stuff is just descriptive fluff that you could apply to any class if you wanted to.

 This is something I came up with, but I’m just not sure if it’s mechanically viable

Quote from Chicken_Champ >>

Agree that Divine Soul Sorcerer can do a lot of the same spell selection and healing focus as a Cleric. But also, just being a Celestial Warlock can pack that multiclass into a single class as well, giving you your usual Warlock invocations, spell selection, and Pacts but also some limited cleric spells, a healing feature, and some holy-type features at higher levels. A class that can both cast Cure Wounds and Arms of Hadar should be able to do a good amount of work telling your narrative story of duality, without having to mechanically multiclass.

However, it sounds like you may already be a Life Cleric, and more interested in how to add in a Fiendish element to that than vice versa. From that, I take it that you probably have good wisdom and con, maybe a decent str to wear heavy armor, and probably low-decent charisma for the warlock multiclass. You probably aren't focused on Strength- or Dexterity-based melee attacks currently, you're probably using Wisdom-based spells and cantrips. Good news is:

• you don't need high charisma, unless you plan on attacking or using save-based spells from your warlock multiclass. The only fiend features that specifically need charisma are your THP bonus from killing enemies (not consistent enough to worry about), and many of the bonus spells that are added to the warlock list for you (which you aren't compelled to take). Instead, you could just learn Warlock spells that don't attack/require a save, and experience no real penalty from low charisma at all, while benefiting from other Warlock features, invocations, and pact boons that don't check charisma.
• your Life Cleric bonuses to healing spells (both Disciple of Life and Blessed Healer ) don't require Cleric spells, they're happy to work with any spell that restores hit points. However... I can't think of many on the Warlock list that restore HP, other than Vampiric Touch, and then we're back into the problem of avoiding Charisma above...

Really... Fiend probably isn't the Warlock subclass you want if you're looking for one to provide you low-Charisma benefits. Just as the narrative need not bully you into a Warlock multiclass, it also shouldn't bully you into the Fiend pact, you can tell that same dual divinity story even if your pact is mechanically Genie, or Archfey, or whatever. Fiend isn't bad, but I'm a big fan of Genie, since it gives you several cool features, and has a bonus that scales on character level rather than warlock level or charisma, which will better help a multiclass dabbler. Celestial is pretty charisma-dependent and probably overlaps rather than enhancing your cleric features and spells... but Undying would be another decent low-cha splash class. 

 Thanks! This really helped. I’m still planning on leveling up a bit as cleric before multiclassing, so will have lots of time to think it through.

If you want to work in some thematically infernal abilities into your cleric... I'd suggest multiclass dipping into Wildfire Druid , actually, not Warlock. You get a summonable little fire critter (which absolutely could be described as a cackling little fire imp), and class features that enhance both fire damage and healing abilities. There's quite a bit of synergy between a Life Cleric and a Wildfire Druid, in my opinion, and they're both Wisdom-based full progression spellcasters. You get access to a lot of the same fire-type spells on your Druid list that Fiend Warlock would have added to your Warlock list, but now they're Wisdom based so actually useful for you! Even if you don't take the Druid to 6, the baseline spirit at 2 is a pretty cool feature which will scale decently with character level (only its HP is really dependent on Druid level). 

Just don't ever use your Wild Shape feature to Wild Shape if you're worried about thematics, just reserve all its uses for summoning the Wildfire Spirit or Wild Companion, and this will look to your group like your an infernal firey summoner, as much or more than multiclassing into Warlock would have! Only real drawback I see is, if your DM wants to enforce the metal armor restriction, you might be forced to wear Splint rather than Plate until you can find some armor made out of dragon bones or something :p

What’s your character layout? Do you have stats, background, lineage, and 1st level already?

Quote from JTArtful >>

My character serves the life-giving sect of a duotheistic religion. However, I was talking with my DM about multiclassing and he suggested I could multi class into a fiend warlock with my patron as the other god, an underworld deity. He has come up with a possible encounter that could spur the change. What do you think I should do? 

Sounds like a cool idea to suit your character! Look at the options that best tell your story and see how it works out. Here's perhaps some invocation ideas:

- Beguiling Influence could step up some of your CHA based skills if you want your character to appear more charismatic following this change.

- Book of Ancient Secrets could extend the rituals available to you.

- Devil's Sight/Eldritch Mind/Fiendish Vigor might reflect the darker side of your god.

- Gaze of Two Minds/Mask of Many Faces seems to fit the duality concept!

I don't know how it is in your game, but I wouldn't worry about optimising if the decision makes your character more inline with your vision.

I agree! Thanks for your advice. I’ve decided I will multiclass, although I think I’ll level up a bit more in cleric before I do.

I think a cleric/warlock could be a fun melee build with Spiritual Weapon, Spirit Guardians, and Armor of Agathys all up and running. Takes a bit to set up, but could be good in a big fight. 

As others have said, play the character you want to play. And if you need more advice When the time come to MC I’m sure plenty of people here can help you out.