This Black Phantom can only be found in Pure Black World Tendency. Killing it shifts your character tendency towards Pure White.
A world reaches PBWT when you die in it in Human Form. So just use a Stone of Ephemeral Eyes, jump off a cliff and repeat. You see the chaning effects only after re-loading into the world (not by dying). So after your death, click on the Archstone again and you can teleport into the same archstone to see the shift towards dark or light world tendency. Luckily, the PS5 doesn’t have long loading screens. :-)
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