The 幻影致命光 is a laser beam that is fired by Terraria's final boss, the 月亮领主 and its creatures, the True Eyes of Cthulhu, though they fire a miniature version. Coming in contact with the deathray causes 150 damage in normal mode and 300 damage in expert mode.
The deathray emerges from the central eye of the Moon Lord as a thick line that reaches beyond the edges of the targeted player's screen, and will sweep the area, covering most of the screen in its path. The only ways to possibly avoid the deathray is to be close to the Moon Lord's head and move around the central eye as it fires the ray, or, since the ray doesn't go through 物块, to be behind solid blocks as it sweeps.
The Moon Lord uses this ability every time he opens the eye in his forehead, though he takes time to charge and shoot the deathray and he also takes time to close it after he's done channeling it.