Mark of Oracles and Where to Find Them?

[OldFartAtPlay]

Good catch, I forgot about the Legendary Crate. I don't recall ever seeing a Legacy offered by Goranthan.

[LodWig]

Buyable sources for an event hero:

• Legendary Crafter's Crate, 2500 Gems, 12 to 16 marks (average 14)
• Celestial Crate, 200 Gems, 1 to 2 marks (average 1.2)
• Gornathan's Epic Legacy items, price varies. If level 70, price is about 80 Gems, average yield upon salvage is one mark (This trick courtesy @Bemxuu).

Salvaging a level 70 or above item yields 0 to 4 marks, with an average of 1 for an Epic and 2 for a Legendary. So TL 120 (or above) is better as the boss gives only Legendary items. More details on average MotOs yield per trial level in current season here.

[OldFartAtPlay]

I've heard TL 130 is best but I'm not certain of that. And if the scarcity of drops wasn't bad enough there's also no guarantee that salvaging a Legacy will yield any MotOs.

[ShotDead]

Salvaging Legacy gear dropped from kills .

And I presume there is no particular location (story level or TL) to farm those

[OldFartAtPlay]

Salvaging Legacy gear dropped from kills or Celestial Crates is the only way I know. You can buy a C crate from Ruby for 200 gems but they only have 1-3 MotOs. (The last one I bought only had one). I believe you get one as Seasonal reward unless you buy the Season Pass where you get a couple more.

[ShotDead]

Kill some fantastic beasts?