The description of the Giant's Might feature for the Rune Knight fighter in Tasha's Cauldron of Everything (p. 45) says that their weapons and armor grow with them, meaning their sword would be large/oversized.
Giant weapons roll double the dice. Would a Rune Knight fighter using the Giant's Might feature benefit from this?
The feature description for Giant’s Might is complete - you don’t have to look elsewhere to learn its effects:
As a bonus action, you magically gain the following benefits, which last for 1 minute:
- If you are smaller than Large, you become Large, along with anything you are wearing. If you lack the room to become Large, your size doesn’t change.
- You have advantage on Strength checks and Strength saving throws.
- Once on each of your turns, one of your attacks with a weapon or an unarmed strike can deal an extra 1d6 damage to a target on a hit.
A little bit of extra damage is baked into the feature, and these are all the benefits of the feature. You don’t have to look elsewhere to learn how this feature works.
The Dungeon Master's Guide contains guidance for the DM can use to create new monster stat blocks:
If you want a full monster stat block, use the following method to create your new monster.
The introduction to the Monster Manual explains all the components of a monster’s stat block. Familiarize yourself with that material before you begin. In the course of creating your monster, if you find yourself unable to make a decision, let the examples in the Monster Manual guide you.
Once you have a monster concept in mind, follow the steps below.
The instructions for creating a monster include the following guidance for bigger monsters, which is where you got the idea:
Big monsters typically wield oversized weapons that deal extra dice of damage on a hit. Double the weapon dice if the creature is Large, triple the weapon dice if it’s Huge, and quadruple the weapon dice if it’s Gargantuan. For example, a Huge giant wielding an appropriately sized greataxe deals 3d12 slashing damage (plus its Strength bonus), instead of the normal 1d12.
This is guidance for the DM to use to homebrew a monster. This has nothing to do with player character class features. This section isn't even giving "rules" about anything. This is telling the DM, "If you want to create a monster, here's some help".
It doesn't say that the items change in size, sadly. It only specifies what is being worn.
I would agree that if it did change the weapon to an oversized weapon, it should gain the extra damage dice.
A "monster" is specified as such, if you wanna use the DMG and Monster Manual for such a ruling. The definition is located in the Introduction of the Monster Manual.
A monster is defined as any creature that can be interacted with and potentially fought and killed. Even something as harmless as a frog or as benevolent as a unicorn is a monster by this definition. The term also applies to humans, elves, dwarves, and other civilized folk who might be friends or rivals to the player characters. Most of the monsters that haunt the D&D world, however, are threats that are meant to be stopped: rampaging demons, conniving devils, soul-sucking undead, summoned elementals — the list goes on.
Therefore, player characters also qualify as "monsters".
It's really up to the GM if the weapon size increased via the feature, but the only other plausible way to get that extra d12 on your Battleaxe is if someone used Enlarge/Reduce.
Here is one of the great things about being a DM. You can chose to be fair and make your own ruling on these types of things. You can spend your game trying to come up with excuses to cheat players.
The feature does what it says. Since the Giant Might ability clearly says "you become large, along with anything you are wearing." Anything you are wearing, this includes carrying, is now a size larger, and we now need to know if this change affects the weapon in some way, Thus, we now need to know what a large size, or over-sized weapon does for damage. The Dungeon Masters Guide has rules for oversized weapons on pages 277-278. It begins by talking about monsters who have oversized weapons and how to calculate for oversized weapons. This should definitely include your currently over-sized (large) weapons, making them one level of over-sized, which by RAW (Rules As Written) doubles their dice.
If your DM makes it all about "oversized weapons only apply to monsters", then do what a guy in the campaign I am playing in did, and chose a "monstrous" race like the Bugbear, Centaur, Deep Gnome, Duergar, Fairy, Firbolg, Githyanki, Githzerai, Goblin, Grung, Hobgoblin, Kobold, Lizardfolk, Merfolk, Minotaur, Naga, Orc, Satyr, Siren, Vampire or Yuan-Ti. All of these are races from the Monster Manual, so all of these would qualify, even by the DM’s twisted interpretation of RAW. In this example, the player literally has a Minotaur with a greataxe.
There is no way you can tell me that when a Large size Minotaur with an vicious, large-sized, two handed Greataxe charges to attack you, that it isn't going to cause massively more damage than usual. Any other answer than "yes" and I call, "BS" !
Finally, the best solution to this mess that gives you large-size weapon damage that is impossible to ignore. Take Smith Tools, or hire a Smith, and make a large weapon. You can also get large weapons for free off dead creatures of large size (bugbear, minotaur, Fiends, golems, etc.). Carry the weapon with you, pull it out, put it on the ground and lean it against your body/leg. When you grow, grab the large weapon that already has the increased damage and use it. Now there is no way, without being the world's greatest hypocrite and cheat, the DM will HAVE to give you the appropriate damage.
You can even then ask, after that combat, what is the difference between the large weapon you brought, and the large weapon you also hold, that grew with you. At this point it is fun to watch people dancing to justify their contrary logic/ruling.
And for real-world value, I know of at least 6 tables that rule it this way, so their are quite a few DMs out there who agree with this ruling, you just have to look for them.
