The Creator oozes with the scum and filth He has deemed you to be in His toxic form. The might of the pestilential deity lays waste to His victims faster than the deadliest of plagues. While the goopy master cannot scratch the hide of His Steel nemeses with His divine Judgment, nor is He a friend of the Psychic- and Ground-types He hath created, His viscous hide protects Him from even the most powerful of onslaughts. The venomous creator even consumes all forms of Toxic and Toxic Spikes in His mighty gullet when He enters the battlefield. Bow before your corrosive maker!
Poison is a notoriously bad offensive typing, it is resisted by 4 types, not including Steel-types being immune to it, while only hitting Grass-types super effectively. However, Poison Arceus has its place in the Ubers metagame, due to its ability to absorb Toxic Spikes and its immunity to Poison. Therefore, it can set up on Blissey with impunity, fearing only Thunder Wave. Apart from absorbing Toxic Spikes, this set plays identical to almost every other Arceus Calm Mind set.
Focus Blast is recommended in the second slot due to its ability to hit Steel-types immune to Judgment for super effective damage. However, there are a couple of other options: notably Flamethrower and Ice Beam. Flamethrower hits some of the Steel-types for super effective damage, except that you lose the ability to hit Dialga hard and Heatran is now immune to both of your moves. Ice Beam, on the other hand, trades the ability to hit Steel-types for the ability to hit the plethora of Dragons in Ubers for super effective damage, most notably Giratina, who resists Judgment with its part-Ghost typing. Sludge Bomb is also a possibility over Judgment if you are enamored with the 30% chance of poisoning your opponent, but the loss in power is noticeable to the point where at +6, you have a small chance to not even 2HKO Ubers Softboiled Blissey.Team Options & Additional Comments >>>
The main draw to using Poison Arceus is to support the team by removing Toxic Spikes, so it's only natural to utilize its potential as a team supporter. With 406 HP and 301 defenses, access to a reliable healing move in Recover, and fantastic support moves, Arceus can fill this role very well. Will-O-Wisp is a great move that helps Arceus and the rest of its team against physical attackers, while the final slot either prevents your opponent from setting up, or turns Arceus into an impregnable wall.
Will-O-Wisp is the preferred status move, although Toxic and Thunder Wave both have their merits. However, Will-O-Wisp's ability to soften hits from Groudon, who would otherwise stomp all over Arceus makes it the better choice. Ice Beam's utility and coverage in Ubers makes it the primary choice for an attack, but Flamethrower is also an option, especially if you choose Toxic or Thunder Wave, as it still gives you a chance at burning Groudon and other physical attackers.Team Options & Additional Comments >>>
Flamethrower or Ice Beam can be used over Focus Blast on the Calm Mind set, but the coverage Focus Blast gives is significantly better. Sludge Bomb is a possibility over Judgment if you really want to poison your opponent, but the power loss is rather significant.
For the Support set, Judgment can be used over Ice Beam or Flamethrower if you really desire a STAB move, but having a Poison-type move as your only attack is very suboptimal considering the abundance of Steel-types in Ubers. Thunder Wave or Toxic can be used over Will-O-Wisp, but Will-O-Wisp's ability to cripple Poison Arceus's biggest threats in Groudon and Garchomp makes it a great choice.
Checks and Counters
Groudon is the best counter for Poison Arceus; the standard Rock Polish Groudon will always OHKO Arceus with Earthquake, unless you are running the Support set and are able to Will-O-Wisp it. In a similar manner, Garchomp can pose a threat if Arceus lacks Ice Beam or Will-O-Wisp, as Choice Scarf Garchomp will 2HKO it with Earthquake. Heatran and other Steel-types are a pain for Arceus because it will have to rely on Focus Blast's shaky accuracy in order to take care of them. Steel Arceus deserves a special mention because not only can it Calm Mind along with Poison Arceus, but it could also use Roar to phaze your Arceus out, putting you in a real tough spot.