The stump of a unicorn horn.
On activation, Isaac will start flashing while a distorted, broken-record version of the My Little Unicorn theme for the duration of the effect. Isaac will become invulnerable, but won't be able to fire tears and does not deal contact damage to enemies on his own. Items that activate on contact with enemies will still do so, however. Additionally, Mom's Knife will still hurt enemies.
Due to their similar effects, the Unicorn Stump can be used in a manner akin to the My Little Unicorn for infinite Blood Bank uses, Devil Beggars, Sacrifice Rooms and Curse rooms.
Unicorn Stump synergizes well with all on-contact effects and does not block them in any way.