I think you may be looking for Digimon Tamers.
Takato Matsuki, a fan of the Digimon card game, finds a Blue Card, which transforms his card reader into a D-Power. His original Digimon creation, Guilmon, materializes into real life when his D-Power scans his drawings. Takato meets Henry Wong and Rika Nonaka, two other children who are partnered with Terriermon and Renamon, as well as Calumon and Impmon. As wild Digimon began roaming Shinjuku, the Tamers defeat them and defend the city. Using their D-Powers, the Tamers can Digi-modify through scanning cards or help them Digivolve. After each Digimon is defeated, their Digimon obtains their data.
Hypnos leader Mitsuo Yamaki attempts to send Digimon back to the Digital World. The Tamers began working together with Hypnos when the Devas invade the Real World. Calumon is captured, and the Tamers enter the Digital World to save him. When Impmon betrays the Tamers and kills Leomon, Jeri Kato falls into depression. After resolving conflicts with the Digimon Sovereigns, the Tamers learn that the Digimon are protecting themselves from humans and the Real World after the Digital World is invaded by the D-Reaper, a rogue clean-up program. As the Tamers return to the Real World, the D-Reaper kidnaps Jeri, manipulating and trapping her inside the body. When the D-Reaper begins to materialize in the Real World, the Tamers defeat it, using the program and saving Jeri. With both worlds restored, the children are forced to say goodbye to their Digimon partners, when they end up returning to the Digital World by the effects of the program. The series ends with Takato discovering the portal in the tunnel under his hiding place.
Hypnos is the cult. Impmon becomes Beelzemon and starts draining the other Digimon.
Impmon is granted the power to become Beelzemon, and blinded by the power, goes down a murderous path. He begins destroying virtually anything that comes in his way, eventually killing Jeri Katou's partner Leomon, triggering a battle that nearly ends in his destruction as retribution for Leomon's death. He is spared at the last moment by Jeri, her reasoning being that while she hated him for what he did, she didn't want to see anymore lives lost. Shaken by this display and considerably weakened from battle, he wanders off, now guilt-ridden and ashamed of his horrific deeds. He de-digivolves into Impmon again after being attacked by a group of Chrysalimon, whom he allows to attack him because he no longer wanted anything to do with the power that he formerly so reveled in, and is essentially left to die. Luckily, he is found by Renamon and Rika Nonaka, who decide to bring Impmon back into the Real World. After being not only spared, but saved by the people he had once hated and tried to destroy, Impmon eventually resolves to try and make right all the wrongs he'd committed towards others, especially Jeri and the other Tamers.
The boar mon would be Vikaralamon.
Found by scanning the TV Tropes Mon page, and vaguely recalling Digimon Tamers as having some uncharacteristically dark storylines, then looking around the wikis to find confirming details.