Without any other champion enemy appearance modifiers the chance increases from 5% to 20% or 20% to 35% in hard mode.
Notes
While more champions likely mean a tougher run, players will also profit from a higher amount of pickups available to them, thanks to many champions dropping items upon death.
Champion enemies don't drop consumables while playing Greedier mode, making Champion Belt significantly worse in that mode.
The belt does not affect the appearance rate of champion bosses.
This is one of the only raw DMG up's in the shop item pool.
The chance for champions to appear does not increase in The Void.
Champion Belt's quality is treated as 2 for the purposes of Bag of Crafting.
Interactions
Purple Heart: Champion Belt's increased chance is also doubled.
20% to 35% in hard mode.
The chance for champions to appear does not increase in 
The Void.
Champion Belt's quality is treated as 2 for the purposes of 
Bag of Crafting.
Purple Heart: Champion Belt's increased chance is also doubled.
Concept drawn for Champion Belt.
The Binding of Isaac: Rebirth
