Lost Sectors where you can find Nightmares in the EDZ, IO, Titan and Nessus.
Note: These locations are the only locations i have personally confirmed spawn nightmares, If you know of any others please comment below.
European Dead Zone (EDZ) : Widows Walk

In the area of the Trostland landing zone to the north west of the church there is the Windows Walk lost sector. There are 3-5 nightmares inside making this the best Lost Sector to clear.
Io : Grove of Ulan-Tan

Titan : Methane Flush

Nessus : The Rift

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