Arisselia

It may at first seem like a strange idea to choose Ariados as a candidate to be fused with Cresselia; after all, it is often viewed as a frail, forgettable Pokemon because it is not particularly strong. However, there are several reasons why this Pokemon is instrumental to the capture and defeat of Deuxrai.

Firstly, half of the Ariados species possess a wondrous ability; Insomnia. Pokemon with Insomnia are immune to Sleep, and therefore, all sleep-related attacks. By enhancing that ability, it could perhaps become more than just an immunity...

Secondly, Ariados' typing comes to mind. Though Poison is weak to Psychic, the Bug type resists both Dark and Psychic. An Ariados-Cresselia fusion would only take normal damage from Deuxrai's attacks, less considering Cresselia's ample Hp, Defense, and Special Defense, and its ability to heal itself. Also, it would be able to strike back with STAB, 4x effective Bug type attacks.

Furthermore, Ariados' innate hunting abilities will only aid in the search for Deuxrai. Ariados is a voracious nocturnal predator, using silk to net their prey with or track them straight to the nest. These instincts coupled with Cresselia's natural tendency to look for and battle Darkrai will make this fused Pokemon an excellent tracker.

The fusion of Cresselia and Ariados (coined 'Arisselia') will likely result in a Pokemon with poor Speed and Special Attack, good Attack, excellent Defenses, the ability to nullify Sleep-related attacks. Arisselia and Deevox could be the combination that seeks and destroys Deuxrai once and for all...

(EDIT: I made it into !!! Thanks so much, !)

(EDIT 2: A big thank you to everyone who faved and commented!! I really appreciate it! )