Heart Locket

"By the way, Sans...
That heart locket doesn't belong to you..."

— Cross' wham line to Classic!Sans after he corrupts Underfell!Sans.

The heart locket is an object important to the XTale and also the timeline that Sans lives in the main universe. Canonically in Undertale, the locket symbolizes the friendship between the fallen human and Asriel, but in XTale the locket instead symbolizes the bonds and connections between the two children of XGaster, and that of the other monsters.

XTale!Asriel, XTale!Toriel, and XTale!Muffet were given the heart locket from XFrisk, while XTale!Asgore got his heart locket from XChara. The heart locket given to XTale!Undyne was the result of XTale!Alphys giving her the fixed heart locket that the two brothers broke in rage.

After their merger, the X-Event proceeded to give the heart locket to XTale!Mettaton, XTale!Papyrus, and XTale!Sans, though as they continued giving heart lockets, their anger and frustrations against XGaster grows against their desire for freedom, as shown with Papyrus. Cross keeps his heart locket even after he's thrown out by X-Event!Chara, but both of them end up losing the locket after they die. XGaster proceeds to return the heart locket back to XChara once he overwrites to bring him back to life.

In Underverse Season 2, each of the XTale characters (except XGaster and Alphys) now have a heart locket of their own.

In the timeline that Sans lives in, the locket represents pain and suffering, as the way he got the locket was through glitching his own world and causing him to be part of a new reality.