Colossal Creatures: An Optional Rule to Make Monsters More Massive in D&D 5e
Some monsters are bigger than others. To represent size, a creature's token on a battlemap is a certain number of squares wide and tall. Tiny, Small, and Medium creatures are 1 square, Large creatures are 2x2, Huge are 3x3, and Gargantuan are 4x4. This means that all Gargantuan creatures take up a space that's roughly 20 feet by 20 feet.
But what if we want to go a wee bit... bigger?
[Image ID: Steve Buscemi in Spy Kids 2. End ID.]
What's the worst that could happen?
It's not an optional rule to make creatures bigger than 4x4. It says as much in the Dungeon Master's Guide that Gargantuan creatures can be bigger than that if you want. However, I don't think this is quite enough. An 8x8 creature would be twice as big as a normal Gargantuan creature but would have the exact same size of hit die (d20). That doesn't seem quite right to me. Even if we wanted to, there are no bigger dice for us to use for the hit die. Well, we could use d100s but I think that would get messy quickly.
Therefore, I propose that Colossal creatures make a comeback with this optional rule: for each square a Colossal creature is bigger than Gargantuan (i.e. a 6x6 is two squares bigger than a 4x4), it gains a colossus level (so a 6x6 is a 2nd-level colossus). For each Colossus level, it gains 5 points of flat damage reduction. This means that a 2nd-level colossus ignores the first 10 points of damage it takes from any one source, which means that if you deal 32 damage to one you actually only deal 22.
Obviously, casting reduce on a colossus reduces its colossus level by 1 and therefore the damage reduction also decreases by 5. And vice versa!
For the sake of example, I've might add a “Colossal” trait to a roc's stat block and changed its creature type from "Gargantuan Monstrosity" to "2nd-level Colossal Monstrosity".
Colossal. The roc is a 2nd-level colossus. It ignores the first 10 points of damage (after applying vulnerabilities or resistances) from any one source of damage, taking the rest as normal.
Optional Optional Rule: Weak Spots
Just for fun, you might want to say it's possible to ignore a colossus' damage reduction if you use a specific damage type (or types) against a specific part (or parts) of its body. For example, a roc's weakness might be bludgeoning damage applied to its beak! In these cases, the weak spot would be treated a bit like an object with its own AC that the attacker must be in range of to hit. The AC equals the creature's plus its colossus level (the above roc's beak would have an AC of 17). The trait might look like this:
Weak Point. The roc’s beak is a Medium weak point with AC 17. A creature that deals bludgeoning damage to the beak ignores the roc’s Colossal trait and deals the same amount of damage to the roc.
How big is too big?
[Image ID: a diagram showing a set of tokens on a grid that shows how big a colossal creature can get: 9x9 squares. End ID.]
I wouldn't go bigger than 5th-level colossus myself unless I wanted a truly massive creature. 25 points of damage reduction on every attack is good enough for most creatures to be quite difficult to kill.