Mythical item borders look like this:
If you try to upgrade a level 50 mythical item, it shows this:
The relics are used for the ascension. The type of relic depends on the mythical item's transform range. For instance, if you have an Epic-Mythical torso like this one, you need 4 Epic Ascension Relics. Common-Mythical items need 2 Common Ascension Relics, Rare-Mythical items need 3 Rare relics, and Legendary-Mythical items need 5 Legendary relics.