Mercure's Lab (unlocked by clearing Nightmare 5-30) is a hero reroll system: you select five hero units as materials, and their combined Hero Grade Points set your Lab Grade - which controls the quality of the five-hero pool you're offered.
Hero Grade Points
- L+ hero = 200 points
- L hero = 150 points
- R hero = 10 points (Leah also counts 10)
The only two Grades that matter
Grades between the breakpoints barely move your odds - if the goal is better heroes, jump straight from one breakpoint to the next instead of drip-feeding materials.
- Grade 6 = needs 381+ points. R heroes are removed from the pool and L++ heroes enter it. Cheapest combos: 2x R + 3x L (470 pts) or 3x R + 2x L+ (430 pts).
- Grade 17 = needs 801+ points. L heroes are removed, leaving an essentially all-L+ pool with a small L++ chance. Cheapest combos: 3x L + 2x L+ (850 pts) or 1x R + 4x L+ (810 pts).
What the odds look like
At Grade 6 the pool is roughly three-quarters L with about a 26% total chance of L+ appearances and under 1% for L++. By Grade 17 L heroes are gone - you're picking among L+ units with roughly a 1.5% total shot at an L++. As of May 2026 the pool holds 5 L++, 43 L+, 25 L and 37 R heroes (110 total).