The Shadowsphere is an artifact in Heretic. It looks like an ethereal green sphere with a face. Activating the Shadowsphere gives the player the Heretic ghost attribute for 60 seconds, becoming translucent and gaining immunity to non-magical attacks (Such as the Firemace's attacks). As with Doom's blur artifact, it also decreases the likelihood of idle monsters noticing the player, and causes their aim to become erratic.
The Shadowsphere will make the player invulnerable to the axes thrown by Undead Warriors, so it is particularly useful when facing them. Note that the Undead Warrior's melee axe will still cause damage.