The Undying Ones present themselves as beautiful, at the prime of their lives and power, but actually are ancient and withered, their flesh violet blue and even their nails and white of their eyes have turned blue for drinking the shade-of-the-evening.
A Clash of Kings
Following the suggestion of Pyat Pree, Daenerys Targaryen seeks the Undying Ones counsel at their residence, the House of the Undying. After a long search, Daenerys finds a splendor of wizards in their prime: young, strong and beautiful. They offer her knowledge and magic weapons. Drogon, however, points her to another door. Daenerys abandons these wizards who call on her with sweet voices. She runs and finds a gloomy chamber with a stone table with a blue and rotting human heart floating above it. In the darkness Daenerys perceives the true Undying Ones: ancient and decrepit old men and women with ripe blue-flesh, lips and eyes, unmoving and unbreathing but still whispering to her. The Undying Ones show her a number of visions of past and future events, but as they do so they attack her and attempt to drain her life. If not for Drogon, they would've succeeded.
A Dance with Dragons
"When you come to the chamber of the Undying, be patient. Our little lives are no more than a flicker of a moth's wing to them. Listen well, and write each word upon your heart."
—Daenerys thinking upon the Undying Ones in Meereen