The BIS relics at the moment are any ones that give Lava Imbued. Note that this trait does not appear on any relic from Nighthold so in order for any of the pre-Nighthold relics to actually be BIS they need to Titanforge high. Lava Imbued is worth roughly 4 weapon item levels above the relics that don’t improve single target (which most relics in NH don’t).
Lava Imbued Relics can be found from Eye of Azhara, Blackrook Hold, Ursoc and Ilgynoth at least. There is probably some source I’m forgetting. So to get them you would need to do Mythic Emerald Nightmare and hope it titanforges or do a +15 mythic and hope you get a good one in your weekly chest.
Note that when 7.2 comes we’ll get a new trait that might become better than Lava Imbued so BIS might change then.