Can You Put Density and Breach on a Mace? No, Pick One

Why not

Density and Breach can each go on a mace, just not together. Here is what a mace can take instead: Curse of Vanishing, Wind Burst, Smite, Bane of Arthropods, Density, Breach, Fire Aspect, Unbreaking, Mending.

What that means in practice

Both are damage enchantments for the mace, and they also exclude Smite. Density adds damage for every block you fall, while Breach weakens the target's armor. Wind Burst and Fire Aspect fit with either one.

Which one to pick

Density for mobs and raw damage, Breach for armored targets like players in netherite.

Mob farms and PvE. Density. Most mobs wear little or no armor, so raw damage wins. A Wind Burst bounce turns every hit into a new smash.
High drops and pillar fights. Density. Its bonus has no cap on height, so a long fall onto a target hits extremely hard.
PvP against netherite. Breach. Netherite armor and toughness soak up a large part of every hit, and Breach IV cuts that armor's effect by 60%.

The Density vs Breach comparison goes through every situation, with the numbers for both.

Why the game says no

Both enchantments can go on a mace on their own. Each one lists the other as incompatible, though, and that is enough to block the pair. If you try with a single conflicting book, the anvil shows no result at all. Also, the enchanting table never offers both in one roll.

Java reads these rules from each enchantment's data file, which lists the enchantments it can't share an item with. The full list of clashes is on the incompatible enchantments page.

The max build with each

With Density, a max mace has Wind Burst III, Density V, Fire Aspect II, Unbreaking III and Mending. In the cheapest order it costs 31 levels over 5 anvil steps, and no step goes above 10. Open it in the calculator.

With Breach, a max mace has Wind Burst III, Breach IV, Fire Aspect II, Unbreaking III and Mending. In the cheapest order it costs 34 levels over 5 anvil steps, and no step goes above 11. Open it in the calculator.

The best mace enchantments page explains every book, and the mace enchant order shows each anvil step.

Where Density and Breach go

Density goes on mace. The enchanting table can offer it there directly.
Breach goes on mace. The enchanting table can offer it there directly.

Commands and creative mode

The /enchant command refuses the second enchantment, because the pair is incompatible. Only /give with the enchantments component can create an item with both. That is a command-only item, and survival can never make it. The command generator writes the right syntax for your edition.

Java and Bedrock

As far as the item list goes, the answer is also the same in Java and Bedrock. The anvil differs, though: Java charges the full new level of every book. This page follows Java 26.3.

About Density

Density makes the mace's smash attack hit harder the further you fall. Each level adds 0.5 damage for every block fallen.

Density has a max level of V. If you need a book, you can get it from the enchanting table, librarian villagers, ominous vaults (trial chambers) and enchanted loot from fishing and chests. A librarian sells a Density V book for 17 to 64 emeralds. Before you buy, read everything about Density, including what each level does.

About Breach

Breach lets mace hits ignore part of the target's armor. Each level reduces how effective the target's armor is by 15%.

Breach has a max level of IV. If you need a book, you can get it from the enchanting table, librarian villagers, ominous vaults (trial chambers) and enchanted loot from fishing and chests. A librarian sells a Breach IV book for 14 to 58 emeralds. Before you buy, read everything about Breach, including what each level does.

What else a mace can take

Besides Density and Breach, a mace can take Wind Burst, Smite, Bane of Arthropods, Fire Aspect, Unbreaking and Mending. Of those, Smite and Bane of Arthropods conflict with Density and Breach. Then the best mace enchantments page ranks them and gives the max build.

Questions

Can you put Density and Breach on a mace?

No. You can't put Density and Breach on a mace together. They are incompatible in Java 26.3, so a mace can only have one of them.

Which is better for PvP, Density or Breach?

Breach against players in strong armor. Density against mobs, which wear little armor.