Romantically, I see a combat between a massively large creature and a group of small humanoids rather differently than it tends to play out in the campaign. I see the humanoids rushing forward—quickly finding themselves in over their head and being forced to scatter for their lives. The lumbering hulk of a creature peering behind rocks or trees; tearing the trees up by their roots and tossing them away. A member of the party stepping out and firing a bow. The monster bounding across the distance, making an attempt to smash at the bowman...missing or hitting, but now being close enough for another, nearby fighter to make one desperate swing. The mage, terrified, moving from rock to rock, waiting for the chance to let go a spell (that won’t be that effective, but will distract the monster for a round or two)...
Most of all, the battle going on, and on...
What happens, however, is that the party swarms the monster. Even a hydra attacking with 10 heads isn’t really enough to dissuade a seven-person party (characters and henchmen) with 5 or 6 levels. There’s no tactics to it...just leap it and keep swinging until its over. The monster barely has a chance—it gets in one deadly hit per round, then finds a third of its hit points gone from magic, swords and missiles.
I want a monster to be so terrifying that the party, in the encounter, realizes A) my puny sword isn’t going to do much; and B) I really could actually die.
This is the purpose of the mass table. The greater the mass, the more damage it can tolerate, as according to the shown table. Average hp per “die” gives a sense how many hp a truly large monster might have. You might notice that the monster’s mass goes up considerably faster than the average hp/die...this was done on purpose, simply to control the potential number of hit points and also because it is a fair argument that larger monsters carry more fat and other non-muscular structure as part of their overall bulk, and therefore should be penalized for this in terms of their combat resilience.
(The "average" and the actual dice roll do not agree because I wanted to keep the desired average separate from that of the dice so that, should I decide to change the dice, I have the original calculation. Don't worry about it, its nerd thinking)
A mastodon, which I understand weighed about 8,000 to 12,000 lbs. Call the average 10,000, or five tons—on the table, that means a mastodon would have an average of 43.3 hit points/die. With its 12 dice, that’s a total of 519.6.
Ridiculous, you say? Well, why? I threw a mastodon at the party with 102 hit points and it went down in four rounds (yes, one of those rounds included an 8-die fireball). This mastodon, therefore, should last twenty rounds, even thirty or forty, given that the party’s fireball could only be used once.
I tell you, there’s no doubt in my mind that the party would think twice about tackling one.
Okay, yes I know, the table includes things like a “d42.” This is because I can make a die of any size on my excel. I wanted there to be averages for each die but I did not want a high minimum, such as 28d6 would have produced (in exchange for 4d42). If you don’t use excel, substitute your own figures.
And hell, change them all. Downgrade them considerably...its your system. If you like the idea but find this just insane, you don’t need to go to my extremes. The idea should suffice.
My difficulty is in getting good numbers for monster masses. For example, lets take humanoids:
This is basing humanoid weights on the human template given in the DMG: 175 lbs., 6 feet tall. Yes, the numbers generated don’t match up with the Monster Manual...oh, boo hoo. Suddenly an elf with 1+1 HD is reduced to 2-7 hp. Cry me a river. Meanwhile, the orc, listed in the book as 6+ feet, gets a d10. Which might help explain why they’re a little scarier than another human.
But if you think the creatures are all too small here, consider the same table, generated using a dwarf template:
I know this won’t be a popular idea. I have yet to try it out on my party with something really large. It should be fun, when they’ve done 250 hp damage and the fucking thing just keeps on coming.