So I have completed the game on Ultimate (all the quests and bosses killed except for Tomb of Heretic superboss, as he’s unkillable on the level of Ravager).
How do I farm for better gear? I need some mythical legendaries and recipes, namely mythical black matriarch ring, mythical warborn set, runeguard greaves etc.
Also I want this armour for transmog, are these random legendary drops? I have both loyalist pack DLCs and have no such transmog, so I assume it is:
Run rogue dungeons, play Crucible mode (extra DLC that’s a battle arena where you fight waves of foes) or dive deep into the Shattered Realm. Most drops in the game are random.
I don’t know what set that is, but some are not able to be transmuted into other pieces.
Totems and Skeleton Key duneons are good for gear farming. SR makes profit only in high shards (above 60). Crucible 130-150 Gladiator is also nice, but it is not for everyone’s taste (I would prefer SR, for example).
The set drops in Morgoneth dungeon only. Helmet from boss itself, gloves/boots/pants are from Scions of Hunger, shoulders from Opulent Chest guarded by two Magi and chest from Forgotten Reliquary as a reward after surviving special event with waves of monsters (do not recall the name).
Recipes of conduits (special amulets) for all masteries randomly appear in Tomb of Heretic trader assortment, Recipes of helms - in Ancient Grove trader assortment.
If your character has moderate damage/sustain and is not very suitable for roguelike dungeons farming, then the well-known sweet spots for farming are Smuggler’s path (boss runs) and bees (that is, Stirring Hive and Swarming Hive). Runs to these locations are fast thanks to close rift and rather easy, which results in good loot per minute. Farm in shattered realm and crucible even better, but requires a solid character.
I cleared Ancient Grove rather fine but had to use 8 second invulnerability crystal on boss. Also killed most nemesis monsters except for big red demon with buff removal.
And there was warborn visor recipe, but it wasn’t mythic. Is that random and I can get mythic recipe as well? Also how do I get other set pieces, just random drop?
Where is Morgoneth dungeon located?
It is called Tomb of the Heretic (north part of Forgotten Gods expansion map).
Oh, that’s bad.
I know this location, the final boss has as much HP as Ravager and deals huge damage, I was unable to even kill him once for a quest (other rogue like dungeons final bosses are easier).
Ravager is much, much healthier and deadlier Ravager of Flesh - Grim Dawn Monster Database, compare to Morgoneth Grand Magus Morgoneth - Grim Dawn Monster Database 
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On Absolute difficulty after certain character level the trader in ancient grove should offer mythical versions of the recipes. Other set pieces can be dropped randomly or you can craft several helms and transmute them to other pieces (after performing Kargon’s quest in Conclave of the Three).
EDIT: indeed, trader in ancient grove sells only non-mythical versions of helm recipes. My bad.
IIRC, random drop recipes roll based on what recipes you have not unlocked yet. So if you haven’t unlocked warborn visor, you can get either normal or mythic. now that you have normal unlocked, the next warborn visor recipe you will get will be mythic. There’s no specific order as far as I know.
This does not apply to recipes you gain as rewards or from vendors, they can be duplicates.
It is true that only recipes that you don’t have already drop. But this is simply because the others are made invisible. In a way, they still drop, they are just not shown, which makes getting blueprints in Grim Dawn increasingly more difficult and players really struggling with getting those last ones.
Whereas if it was truly as you said, not the game simply hiding the blueprints you already posses from you, I mean the game drawing from the pool of blueprints that you don’t have, we’d would have a constant rate of blueprints drop, not a diminishing one and no problems with getting last blueprints.
Yeah, it looks as if we didn’t get duplicates but in reality we do and the decreasing number of blueprints you get per unit of time matches that, they just not shown because they wouldn’t be readable anyway.