Crystal Triangle

Crystal Triangle ? is a 1987 Japanese OVA.

Koichiro at first wishes to simply ignore the harbringer of doom, but is motivated to action by The Old Man of Hakodate a shamanistic soothsayer that lives in the dump and gives wisdom to all that seek it. Joined by Isao Murakami, a young delinquent fresh out of juvenile hall, the two set off on an epic journey across South Brunswick city park, to obtain the last shard of the crystal triangle from the great monster Ginji Iwagami, a mutated albino alligator that lives in the sewers. The two manage to bring down the monster, but Isaos soul is trapped within the shard, leaving his body as little more than a mouthpiece for the will of the Crystal Triangle.Koichiro begs the old man of Hakodate for a way to save Isao, who presents him with the terrible choice, Surrender the last Shard back to the triangle, releasing Isaos soul, but also returning the Triangle to its complete, immortal state, or kill Isao himself, thus preventing the Crystal Triangle from ever being completed, saving the rest of civilization from the terrible doom it represents. As the final hour grows closer and closer, Koichiro is forced to weigh the uncertain fate of many against the definite fate of one. ........

Source: Wikipedia