Seriously though, don’t open this post if you want to find them by yourself. Don’t ruin a good surprise.
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But if you couldn’t care less about that, here you go.
I managed to catch three mythic fishes by now. Maybe by three they come too, maybe there are more.
Molten Martyr, Ghoulworm and Fang of Tathamet
Molten Martyr (Lava)
First one is obvious as everyone knows by now that you can fish in lava. And it’s the easiest to catch, because apparently it’s the only fish that lives there and it’s a guaranteed drop. At least for me lol: I tried 11 times and got 11 Molten Martyrs. It’s 10 now, I ate one.
I think it’s intentional, so players are aware of such things and start wondering if there is more to it. You don’t even have to go to Scovos or anywhere far really. There are convenient rivers of lava just to the southwest of Cerrigar. Nearest waypoint is Fate’s Retreat.
Ghoulworm (Blood)
This one is a friendly neighborhood fish too. Puddles of druid blood in Shrouded Moors, to the east of Cerrigar. Waypoint — Tirmair.
Fang of Tathamet (Helltide)
This ugly mf was actually my first catch. Imagine my face when I idly wondered what kind of trash I could fish from Helltide and got that purple beam. I got his ass in those small ponds in the frozen tundra just outside of Kyovashad, near Sir Lynna and his infamous Darkblade. But any Helltide with a body of water should work, I think. Not sure about coastlines though. Edit: coastlines are working, confirmed by u/Chuck0709.
All three give you separate Challenge Achievements, but they don’t appear in the Fishing section, they go under Feats of Strength.
Also, I tried fishing in the pools of whatever that is in the Temple of Rot, on the Black Lake, in the intestines of that giant demon’s remains in Nahantu, and in the PVP zones (in water and lava both). No dice, you can’t fish there. I'll try NMDs next. Edit: There IS something to fish in NMDs, confirmed by u/Remarkable_Dot7915.
If you were lucky to find another purple freak or have ideas about where they can be — please share.
PS: Drop rates are fairly high, you can catch a bunch and put them in the stash just in case there’s some kind of horadric cuisine we don’t know about yet.