House Rule: Behemoths

(Created by astakhan937 on r/DnDBehindTheScreen)

What is a Behemoth?

Whether something is considered a Behemoth depends on who or what is fighting it. Any time you are fighting a creature that is (a) Huge or larger, and (b) at least 2 size categories larger than you. Thus, a Frost Giant (Huge) would be a Behemoth to your run-of-the-mill party of small-to-medium adventurers. If you cast enlarge on one of those medium adventurers, the Frost Giant would no longer be a Behemoth to that character, but would remain a Behemoth to the others.

Fighting a Behemoth

• When you hit a Behemoth with a weapon attack or a cantrip with an attack roll, they are treated as having resistance to any resultant damage, unless you have advantage on the attack roll.
• When casting a cantrip with a saving throw against a Behemoth, they have advantage on the saving throw.
• When subjecting a Behemoth to an effect that would knock them prone or otherwise physically move them, if that effect would allow a save to avoid, the Behemoth has advantage on that save. If the effect normally does not allow a save, the Behemonth can make a Strength save to avoid it.

Deadeye Shot

When making a ranged attack roll against a Behemoth, you can choose to either 'Aim For Centre Mass' (thus being subjected to the rules for Behemoths and damage resistances), or you can make a Deadeye Shot - forcing you to aim for a higher AC, but nullifying the resistances the target gets for being a Behemoth. This higher AC is usually +5, but this may be higher or lower depending on the enemy. For example, the armoured hide of an ankylosaurus or ancient dragon makes finding an weak spot much harder than the relatively exposed form of a hill giant.

Grappling / Climbing

You cannot ordinarily grapple a Behemoth. Instead, you can make an appropriate Ability check - usually Strength or Dexterity, and usually Acrobatics or Athletics - to climb onto the Behemoth. Once you have climbed onto it, you can move around on it as though it were difficult terrain and, so long as you are climbing on the Behemoth, you have advantage on any melee attacks against it.
Of course, the canny adventurer will know that being attached to a giant monster as it flails around in combat, or as it is killed and falls to the ground, can be dangerous to ones health!