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Decarabia is a Persona of the Fool Arcana, and you’ll need to fuse one if you intend on filling out the Compendium inPersona 3 Reload. Even if you choose not to use Decarabia in your battles, it’s a useful Persona to “unlock” since it can be used in various dryad fusions. When you first start out in yourPersona 3 Reloadplaythrough, Decarabia’s fusion won’t be accessible. You’ll need a special key item that’s found on the Tziah Block in Tartarus.
Where to Find the Pentagram Stone in Persona 3 Reload
Decarabia Fusion Recipes
The Pentagram Stone can be obtained bydefeating the Isolated CastleonFloor 143 (Tziah Block). This Gatekeeper Shadow hasno weaknessesand willnull all magic (except Almighty). You’ll most likely not have a Persona that can dish out devastating Almighty damage just yet, so you’ll want to bring Personas that canbuff/debuffanduse Physical Skills. Recommended party members areYukarifor healing, andAkihikoandAigisfor physical attacks. Both Akihiko and Aigis have buffs that can increase your evasion/accuracy and attack respectively.
Once you have the Pentagram Stone, you can start fusing Decarabia in the Velvet Room. Here are several fusions you can try:

More Information About Decarabia
Decarabia Weaknesses & Recommended Skills
Decarabia’s base level is54, which means you won’t be able to fuse it unlessyou’re level 54 or higher. Decarabia has all-around stats, but its Skill List consists of Magic and Passive Skills. It has no resistances, but it isweak to Slash Skills. However, if you manage to raise Decarabia to level 60, it will learn theNull SlashSkill, which negates its only weakness.
35

-
20 SP

24 SP
55
12 SP
56
Halves the time needed to recover from status ailments (ex: Poison).
58
32 SP
60
Nullifies all damage from Slash Skills.
You’ll likely end up with a different Skill Set depending on which Personas you’re using to fuse Decarabia. Despite this, it has access to Megidola, which isa tier lower than Megidolaon. Until you can have a Persona with Megidolaon, Decarabia will be able to target enemies that null most affinities, such as theSupreme Handon the first half of theYabbashah Block.
If you decide to use Decarabia to fuse another Persona, then you may want to consider leveling it up to where it learnsNull Slash. This is a great passive Skill for your other Personas to inherit.