The skulls icons on the map are where three undead abominations spawns, one on the circular platform, one on the bridge and one in the cave, the 3 monsters have a small chance to drop Plague Ichor.
There is a "big" version like the first boss that have a good chance to drop a legendary 2h sword, and a smaller version that can only drop ichor. (Monster and chest respawn every one hour approx. in real time).
NPC Randolph in Sacrament (the one who gives bounties/quests near the whisper/teleport) sometimes gives you weekly quests that asks you to kill an abomination in the War Room of Sacrament, and an abomination in the Sewers (quick access north of Sacrament), they drop an ichor.

---> https://www.youtube.com/watch?v=eLpaxYo6KLQ