The Heavy Armor Master feat provides a flat reduction in damage taken while wearing heavy armor. This makes it exciting for front-line martial characters who typically endure large numbers of attacks, especially if you’re planning to forgo a shield. The Strength increase further improves this pairing, making it easy to fit Heavy Armor Master into builds relying on big, two-handed weapons.
If you manage to find resistance to weapon damage types, remember that damage resistances/vulnerabilities are applied after all other modifiers to the amount of damage. This diminishes the value of Heavy Armor master significantly, so if you have the Barbarian’s Rage available, Heavy Armor Master isn’t worth the feat.
At low levels, Heavy Armor Master’s damage reduction is massive. Enemies rely almost exclusively on the affected damage types and reducing the damage by 3 will represent a consistently large proportion of the damage per attack.
However, as you gain levels, 3 damage represents a shrinking proportion of the damage you will suffer from each attack. But even then, creatures which rely on weapon damage gain Multiattack almost universally starting at CR 5, and creatures all the way up the CR range rely on non-magical attacks. Seriously, the Tarrasque’s attacks aren’t magical.
The ideal scenario for Heavy Armor Master is a large number of enemies dealing low individual damage. Against large single enemies, you want a replenishable source of temporary hit points like Heroism. A pool of 5 temporary hp each turn plus reducing incoming damage by 6 from a single enemy provides a massive buffer before eating into your real hit points, making you exceptionally durable at little cost.
Of course, this puts you into Tank Fallacy territory, so lean hard into offense with your remaining features.
