Primal Reversion is a Pokémon mechanic where certain Pokémon return to a more ancient, powerful form. In Pokémon GO, it currently applies to Kyogre and Groudon, giving them a temporary boosted state in battle.

What it means

In the main Pokémon series, Primal Reversion first appeared in Pokémon Omega Ruby and Pokémon Alpha Sapphire. It is similar to Mega Evolution, but instead of becoming a new evolved form, the Pokémon reverts to an older, primal state.

In Pokémon GO

  • It works only for Kyogre and Groudon.
  • The Primal form is temporary, lasting 8 hours in Pokémon GO.
  • When activated, the Pokémon gets stronger and also provides bonuses to other Pokémon of certain types during raids.

Simple example

If you Primal Revert a Kyogre in Pokémon GO, it becomes Primal Kyogre for a limited time and boosts Water-, Electric-, and Bug-type related bonuses while it is active.

Forum-style gist

A lot of player discussion focuses on whether it behaves like a Mega Evolution. The practical answer is that it is very similar, but it is its own system with its own energy and form rules.

TL;DR: Primal Reversion is a temporary power-up form for Kyogre and Groudon that makes them stronger, and in Pokémon GO it works a lot like Mega Evolution but with different mechanics.