Any cool synergies / builds anyone has for this?
My last character, I was doing a shadow sorcerer + bard. I had really envisioned picking up four levels of bard to get a school (Lore), thus getting me some expertise + becoming more of a skill wonk. But what happened was that over the course of the campaign, about half the characters ended up dying (at different times) and so by 2/3 of the way through the campaign, we already had people who were multiclass bards. So what I would get at Bard level 2 was never worth what I got as a high-level sorcerer managing to nab higher level spell slots and higher level spells.
(I think we hit level 14? And right before the campaign ender. As it was, the spell and slots I got in that last level jump pretty much enabled the group victory, as it as actually played out.) So really I just ended up being a high level shadow sorc variant human.
----
One effective non-team player build I saw over the course of our campaigns was a half-elf warlock with eldritch blast and elven accuracy, who could cast darkness and had the ability to see through their own darkness. So basically at high levels, when you're getting three EBs per attack, you cast darkness, which lets you roll with advantage... but at that point with elven accuracy you are rolling with double advantage trying to get a crit on each bolt with multiple d20 rolls. I can imagine if you had Sharpshooter, you'd be consistently adding +10 to your damage + if you managed to pick up Fighter: Champion (3 levels of fighter, but you'd have to meet the stat criteria), you could then drop your crit range into the 19-20 range (10% chance, instead of 5%). Coupled with warlock riders to add your CHA to spell damage, etc., you could probably be putting out 30-50 points of damage per round with just a cantrip.
Of course the big problem is that you would be casting darkness on the enemy, which would envelop your teammates as well. So unless they have a way to mitigate, or you are targeting one of a multiple pool of enemies, you're shutting out your party from dealing with the threat except for those who emerge from the dark.
(Note: If you can lock down the targets inside the darkness so they cannot exit it, that's even better. That also applies to other large nasty area of effect spells, such as any warlock spell involving Hadar.)
----
Typically you want to minimize the number of stats you need to boost (i.e., aim away from a "MAD" build). Classes that utilize the same main stat synergize better -- for example, builds using CHA as a spell base, which includes sorcerers, paladins, bards, and warlocks. Paladins love getting spell slots from other CHA classes since they will just use them to smite with. It's always worth determining whether you should just grab a 1-3 levels of one class, the other class, or do a more equal build, and which class is favored can radically change play style. Also, most classes get their ASI (ability score increase) every four levels. If you don't take a multiple of four levels in a class, you could lose out either +2 in ability scores or a feat. So all that gets factored in too.
Mainly discussing 5e right now. Pathfinder is much more feat based and it's common for classes to get feats every other level... and you need a variety of feats to be effective. Here, feats aren't quite as essentially, but there's a handful of feats that can really up viability or open doors for certain types of characters. (for example, 2H fighters getting polearm master + Sentinel .)
but I think it's worth a shot. 
