Also for future reference, since this is one the first search results when searching the Crests effects, here is some details that the Gheist's post didn't have and I searched on Discord:
- Claydol: Merges its defense and special attack stat to be the same. It should also boost Beam-type attacks, allowing it to use Hyper Beam without recharge and Solar Beam without a charge turn. Does it boosts those moves damage?
- Spiritomb: For each of your pokemon on your party fainted, the Spiritomb gets a boost of 30% on its Atk and SpA. And for each of the opponents Pokemon fainted it gainds the benefit of a Leftovers (recover 1/16th total HP end of turn). At max, with 5 of yours and 5 opponets pokemons fainted, it should have +150 on its Atk and SpA and recover 5/16 of its health. Also someone mentioned it also affected by the faints of your ally on double battles. I have no idea if there is some cap on the bonuses.
- Fearow: Increases critical hit ratio by 3 stages but only works if the target is less 75% health. Does anyone knows if this procs at exactly 75% health remaining?
- Seviper: Causes Seviper to hit harder against healthier foes and speed increased by 1.5x. Someone mentioned that the damage boost on a healthier foe is +50%, but I don't know how it scales. Does it calculated as Crest Damage = Normal Damage * 1,5 * (Target's Current HP / Target's Maximum HP) or is Crest Damage = Normal Damage * (1 + 0,5 * (Target's Current HP / Target's Maximum HP))?
Also the Ariados, Dusknoir, Cofagrigus and Relicanth crests should multiply the end stats, already including the EVs, IVs and nature, and not the base stats as the text says.