The Nameless Venom in Fjordur is obtained by mining purple rocks in an Aberration cave in Midgard, near the portals (Vanaheim, Asgard, Jotunheim).

You should first tame a Rock Drake, place it in a cryopod (2 metal ingots, 4 oil, 8 polymer on ARK: Survival Evolved and 4 in Ascended, 5 hide, 10 crystal, 15 fiber) , craft the saddle (225 fiber, 350 hide, 125 metal ingots, 85 red gems, level 75), and then go farm the Nameless Venom.

Rock Drakes can be found in a cave in Asgard (42.04 / 49.21). Go at night with a high-level Desmodus (200+), turn on invisibility, and steal a high-level egg. There is radiation, so you must wear a hazard suit and be cautious around Shadowmanes.

P.S: When Rock Drakes are babies, feed them Nameless Venom; when they are adults, feed them meat.