Brogue Wiki
The spiked iron ball can be whirled at the end of its chain in synchronicity with your movement, allowing you a free attack whenever moving between two spaces that are adjacent to an enemy.

Flails have a base damage of 10-13 and require 17 strength to use without penalty.

Flails will target just one enemy if you're standing still, like a sword. However, if you move with one equipped, you get an attack on all enemies who are adjacent both to your original square and the square you moved to. This does not include monsters that are invisible or submerged, even if you have revealed them with telepathy. This will attack around corners, even if moving into a space where the enemy can't attack you.

The flail's attack pattern is not as useful as that of an axe or war axe, but generally better than a spear. The way it works around a corner is very useful if you have a monster chasing you. If there is a solid dungeon area you can make a loop around you can get several free attacks on a monster each circuit.

Swords: Dagger · Sword · Broadsword — Axes: Axe · War Axe

Polearms: Spear · War Pike — Maces: Mace · War Hammer
Misc: Rapier · Whip · Flail
Ranged: Darts · Incendiary Darts · Javelins