Blackhole

Blackhole is the first slime past the billion-to-one wall, at 1 in 1.07 billion for a Tier S boss-killer in the secret band. The role label matters here: where UFO and Bomber are general attackers, Blackhole is built to end specific bosses, and that specialization is the reason it earns a slot rather than the raw rarity alone driving the decision.

Equip Blackhole as the dedicated boss-burst slime once you reach this depth, and keep UFO or Bomber alongside it as a generalist attacker. The honest framing is that Blackhole is not a strict upgrade over the cheaper secrets, since its role is narrower, but for the hardest content where bosses simply do not die fast enough, Blackhole closes fights nothing cheaper can. After Bomber and UFO are already in the loadout, Blackhole becomes the situational pick.

Until you regularly fight content where boss HP is the bottleneck, Blackhole sits as a flex pick rather than a daily driver. By the time Ember or Ouchy finally lands, the team has both general damage and boss specialization at celestial-adjacent rarities, and Blackhole keeps its slot in the loadout for hard content specifically rather than every fight.