Mace Protection is a custom enchantment that can be put on any piece of armor including armor which has the Protection enchantment already. Each piece which has the enchantment contributes some percentage of damage reduction when being hit with the Mace.
The stats are as follows:
Level 1: 2.5% / armor piece Level 2: 3% / armor piece Level 3: 3.5% / armor piece Level 4: 4% / armor piece. Level 5: 6.25% / armor piece.
This means a full armor set with Mace Protection V would reduce mace damage by 25%. (if you would have taken 4 hearts of damage you'd take 3).
Mace Protection books can only be obtained at level 1 - 4 from villager trades, The prices are as follows:
Level 1: 15 emeralds Level 2: 25 emeralds Level 3: 35 emeralds Level 4: 45 emeralds.
It is half as likely a villager will trade Mace Protection books than any given book.
You must combine 2 Mace Protection IV items ( 2x book or 2x armor or 1 of each) to get Mace Protection V.
All of this allows the server owner to reduce the effectiveness of the mace in a way that feels natural in-game.
