Are hit dice used to add damage?

Question

I know it’s a stupid question, but allow me to explain. I’m a DM with a group of players who all learned how to play at the same time about a year ago, (me included). We got off to a rough start and still don’t know all the rules, but it’s D&D 5e by the way. And we’ve always played in the way that you can use hit dice to heal like you’re supposed to on rests, however, we’ve done something else as well.

• I’ve always allowed my players to add their hit die rolls to their damage rolls on successful attacks.

  Is this a thing?

I’m pretty sure I read it somewhere, but could not find anything about hit dice for damage in any rules. If I just didn’t look for it good enough, my apologies. But if I’m just crazy and thought of some really cool homebrew rule, then I’d still like to know for future reference in Adventurer's League and stuff.

Answer 1

No, it's not a thing: hit dice don't do damage

Hit dice are used to calculate hit points and healing.

Hit Points and Hit Dice
Your character’s hit points define how tough your character is in combat and other dangerous situations. Your hit points are determined by your Hit Dice (short for Hit Point Dice). (Basic Rules, (p. 8))

Damage comes from weapon damage dice and modifiers. See page 48 in the Basic Rules (Free to down load at WoTC) or the PHB (p. 149).

Damage rolls
Each weapon, spell, or harmful monster ability specifies the damage it deals... When attacking with a weapon, you add your ability modifier - the same modifier used for the attack roll - to the damage. (p. 196 PHB)

Example:

Great Axe: 1d12 + Strength Mod.

With a strength of 16, that means a Great Axe does 1d12+3 damage.

Hit dice and hit points are about how much damage the character takes before dying or being knocked out, or about how to heal. Nothing to do with adding damage in combat.

Answer 2

I've played a version of this.

My D&D group played a one-shot in an existing campaign setting and one thing we tried was a 3rd-party subclass for wizards called Order of Hibernation, published by MCDM Productions.

One feature of the subclass is that the wizard can spend hit dice to add damage to a wizard spell. For example, a 10th level Order of Hibernation wizard can cast Magic Missile, and choose to add 2 of their hit dice to the spell, in this example, adding an average of 7 points of damage to the spell. The wizard then no longer has those hit dice available for healing during a short rest.

Note that this is different from the OP's rules interpretation in that in this add-on rule the hit dice used for damage are spent, replenishing on a long rest, rather than being available at all times.

Our observations:

• This was a fun mechanic that gave our wizard a chance to pack a punch.
• It requires diligent attention to the resource economy, since a wizard who is sure they won't need their hit dice for healing before the next long rest can spend them all on damage, essentially for free.
• Using with Magic Missile is particularly effective, since it can't miss and there's no save. Spending 10 hit dice this way, for instance, adds 35 points of damage.

In short, it's an interesting and fun mechanic.