Relic is an attachment card type. Playing a relic attaches it to a player, and equipped relics may be viewed by hovering over the player's avatar. If more than one relic is attached to a player, a UI element will appear specifying their relic count.
Relics description here.
Relic Weapons description here.
Cursed Relics description here.
Playing multiple relics
Playing a relic weapon while another is equipped replaces a previous relic weapon.
Playing a non-weapon relic has no effect on previously played relics or relic weapons. You may equip multiple relics at the same time. Multiple copies of the same relic may also be equipped.