At the event of the Seething Abomination, you’ll find a bright red marker with a demon skull. If you have PvP enabled, head to the marked location and avoid other players, and you’ll find the Seething Abomination lurking around. Start fighting it to get the upper hand.

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With a bit more power, the Seething Abomination is very similar to the Khazra Abomination found at the end of select dungeons. It’s mainly a matter of avoiding its poison clouds since the cooldown on potions added in PvP zones means they can do significantly more damage than normal.
If you need to back off for a moment and wait for your potion to come off cooldown, you can do so if necessary. Otherwise, it’s a matter of learning its standard attacks and avoiding them. In addition to dropping Shards of Hatred, the Seething Abomination will also fall with persistence.