The title alludes to the Nephilim (Shadowhunter) race being half human half angel.
The moment between Alec and Magnus where Magnus puts a finger to Alec's mouth and Alec smiles was improvised by Harry Shum Jr, which is why Matthew Daddario looks so surprised.
The Circle scenes in this episode were shot at the Parkwood Estate in Oshawa, Ontario.
This episode features the first (excluding Valak) non-shapeshifting demon--or at least one not in humanoid form--seen in the series; the demon even somewhat resembled a Shax or a smaller Ravener.