Two minutes in, Lúthien starts to fade.
From the inside, it feels like all her connections to the outside world are being closed out, choked off, until there's almost nothing left of them. What light still reaches her is so dim she can barely see; what sound, so faint she can barely hear. She can't see her own body at all, and the sound of her singing takes on a bizarre quality because the only part that reaches her ears is what her own body conducts; none of it escapes into the air. Enough air reaches her to fill her lungs, but only just; it feels a little like she's constantly slowly suffocating. Where her feet touch the ground, it's solid - too solid, completely unyielding, and only the ground; she passes through grass and flowers like, well, like she isn't there. All her clothing comes with her, and she can still feel that, but wherever her clothes do not cover her body there's nothing, no air, just an unsettling emptiness.
It is, as promised, very uncomfortable. The sensory deprivation and air restriction are part of it, but being mostly removed from the world also turns out to be bad for temperature regulation. If she doesn't spare some attention to keeping her body a consistent and livable temperature, it will cycle unpredictably between too hot and too cold.