Vecna's Mind Lair

Vecna's Mindscape, or the Mind Lair, is a visual representation of Vecna's mind. Via his 'curse' power, Vecna could make other conscious beings hallucinate and bring them into the mindscape.
On March 25, 1986, Max Mayfield entered the mindscape while afflicted with Vecna's curse, but managed to break out of the hallucination and escape to reality. Nancy Wheeler was later pulled into the mindscape while trying to escape the Upside Down, and was terrorised by Vecna. 
The next day, Max voluntarily made herself vulnerable to the curse again as part of a larger plan to defeat Vecna, making herself ‘bait’ to attract his attention. Max began hallucinating again and was stalked by Vecna through her memories, but to her surprise, Eleven also entered her mind via psychic projection. However, Vecna overpowered Eleven, trapping both her and Max in his mindscape. Eleven broke free, and attacked Vecna midway through executing Max. At the same time, Nancy, Robin and Steve attacked Vecna's body in the Upside Down; this disrupted Vecna's concentration and caused the mindscape to dissolve around Eleven, returning her to the mental void through which she entered.