Ring of Nightmares
A simple ring coated in gold and silver
You cannot talk about one ring of nightmares or THE ring of Nightmares, because there are many. They where created in the infernal and abyssal planes by devils or demons respectively and hold various enchantments. A blood magic variant of the nightmare spell is one of those. In this case the ring sends the dreamer to Evanis(the plane of nightmares). The second enchantment of the ring creates a rift in Evanis between the ring bearer and the outsider, so that the outsider can alter the nightmare by his will on some occasion. The third enchantment is a summoning. The bearer can summon the outsider 1/week if they have met in the dreamscape.