Forgive me if this is well known already, but I don't think I've seen it posted here yet.

The lightning mage build is pretty well known, involving adding in two levels of Tempest Cleric for the channel divinity that maxes out lightning/thunder damage. And it's been noted that getting lightning vulnerability is easy with a bit of water. But what I haven't seen is trying to double things with a crit.

So why not mix in a few levels of Rogue? Here's the final split:

Assassin Rogue 3 / Tempest Cleric 2 / Blue Draconic Sorcerer 7

Assassin gives us access to easy crits on a surprise round. Taking Cleric to 2 and Sorcerer to 7 gets us a 5th level spell slot.

The crux of this setup is the Witch Bolt spell. The Assassin feature only works with an attack roll--most of your other lightning spells are saves. Witch Bolt also scales really well (1d12 per spell level).

So here's the math, assuming you're able to get a surprise round and throw some water:

• Base damage: 5d12 (using your 5th level slot)
• Double dice on auto-crit: 10d12
• Channel divinity: 120 (max damage)
• Draconic Sorcerer 6th level damage rider: +4 (assuming 18 CHA)
• Vulnerability: x2 for a total of 248 damage
• Two targets? Twinned Spell because why not

Note that my screenshot was using a 4th level slot and using a Wild Magic instead of Draconic Sorcerer. Mainly there to show that the math seems to work in game as I've proposed here.

Mixing in the Assassin makes sense for the lightning combo build as well: you can initiate combat by throwing water, then you instantly get your actions back to sling spells. And the other Rogue bonus action skills are helpful to get into position and land other attack-roll spells (by hiding first).

The build is viable at every level, too. Levels 1 to 3 are Rogue to Assassin, pretty straight forward. Next are two levels of Cleric, so now you're an Assassin with Sanctuary, Armor of Faith, and Bless. Then on level 6, you start adding in the lightning, and your nova builds from 100 damage (pretty respectable for level 6), going up by 48 every two levels after that.

You've got lots of flexibility here too. Want access to 2nd level Cleric spells instead of 4th level Sorcerer? Go with Assassin 3 / Tempest 3 / Draconic 6. Want more feats? Sacrifice a bit of damage and go with Assassin 4 / Tempest 4 / Draconic 4 to get three feats. Want to get two (slightly lesser) novas per short rest? Go Assassin 3 / Cleric 6 / Draconic 3.

Thoughts? Any other big nova builds that don't require an overly-specific setup?