Percentage Damage is characterized as an attack behavior that deals damage on a percentage basis which is unlike most attacks, which deal flat amounts of damage. It's introduced in Bloons TD 6 with the addition of Alchemist's Unstable Concoction and Ezili's MOAB Hex ability. It is part of the damage types family.
Percentage Damage is represented on the Bloon Wiki as 
, the upgrade icon for Grand Saboteur, which deals percentage damage to blimps as they enter the screen. It was formerly represented by the upgrade portrait for Unstable Concoction in Bloons TD 6, since the upgrade icon for Unstable Concoction has already been taken with the status effect, Concoction 
. At the time, it would create a percentage-based damage explosion upon destruction of a blimp coated in concoction. However, given that Unstable Concoction was secretly nerfed in Version 27.0 to no longer use true percentage damage, the damage types icon for Percentage Damage was replaced.
As stated previously, Ezili's MOAB Hex ability also deals percentage base damage.
Grand Saboteur's ability in Bloons TD 6 and the Future Crystal in Bloons Adventure Time TD both deal Percentage Damage to MOAB-Class Bloons entering the map.
All attacks require an upgrade in order to have the Percentage Damage.
