The Ancient Seals are deviced scattered the Pokémon World to regulate the natural flow of energies emmitted by Pokémons. They also make human-Pokémon fusions impossible due to the energetic restrictions they induce. The only exception to this rule being the Seals Guardians, which have been merged together before the Seals were activated.
Most of the Seals have been broken throughout history. Only three are known to still be active, all of which lie hidden in the Austeros Region.
The Austerian Seals are guarded by powerful human-Pokémon hybrid which have been created for the only purpose of keeping the Seals intact. It is not known wheter other regions which hosted Seals did the same but could be the reason why only the Austerian ones haven't been broken.
During the main story arc of the games, one of the Seals Guardians passes on its duty to the player, however he or she is not merged with the newborn guarding Pokémon.