Don't forget that you're requiring multiple characters to cooperate for this superhuman effort.
I specified that Bend Luck would have to come from an allied sorceror. Vrax has to be a Fiend Pact Warlock to get Fiend's Own Luck. Whether Vrax or the sorceror has the Cleric level and Bard levels is an open question, though the Expertise suggests that Vrax has levels in Bard. Also, she can only try as often as she can take a Rest to renew Fiend's Own Luck, and *all* of the dice maxing is a rare occurrence.
Don't forget that this is Vrax's *life goal*! She will gather any and every resource she can, across all of 5E!
[/QUOTE]But I thought that the basic rule in 5E was that things don't stack in 5E unless specified. So Guidance wouldn't stack with Dark One's Own Luck or Bend Luck or Inspiration. At best you'd take the best roll.[/QUOTE]
There's a rule on page 205, which is specific to multiple castings of the *same spell*, such as Guidance, use the "most potent roll"; a platoon of clerics each casting Guidance gives the recipient at most +4 guidance.
"The effects of different spells add together while the durations of those spells overlap". For example, Resistance stacks with Bless for saving throws. I'd apply that precedent to different class abilities such as Bardic Inspiration, Bend Luck and Fiend's Own Luck.
I imagine damage modifiers stacking, such as +5 damage from 20 STR, +5 from Charger, +2 from Duelling, +2d8 from Smite, +1d6 from Hex and +1 from an enchanted sword. Correct or incorrect? (None of those specifically say "stackable".)
There are explicitly stacking AC modifiers, such as +2 from shield stacking with light armor and with DEX modifier.
5E has far *fewer* stackable modifiers than other editions, and that's clearly a design choice. PHB's index, under stacking, refers to the spell rule on p.205, and to Advantage and Disadvantage (which do not stack; you have the one, the other, or neither.)
PHB says on page 7 that *specific beats general*. Even if there were a general rule in PHB that stat mod and proficiency were the only modifiers on stat checks, then the details of Guidance, Bardic Inspiration, Bend Luck and Fiend's Own Luck would all override that rule.
If there was a memo telling the PHB's contributing authors not to write any spells or class abilities which gave stat-check modifiers, so that NOTHING other than stat and proficiency would apply to those d20 rolls; then Guidance, Bardic Inspiration, Bend Luck and Fiend's Own Luck are glitches in the PHB. If so, when will WotC issue a correction? Is there a no-stacking rule somewhere in DMG?
Usual disclaimer: if you want a house rule that only the highest damage modifier applies, and/or only the highest AC modifier, and/or only the highest class-ability modifier for stat checks, saving throws and attack rolls, then do what works for your table.