Amulet of Mountains and Gnoll Hills

This amulet is made of brass with a jagged symbol of a mountain engraved on it.  Once per long rest the wearer can cast “enlarge reduce” on themself without concentration.  

When under the effect of the spell you feel the Hunger of Yeenoghu.  If you deal damage to an enemy on your turn you may immediately use your reaction to deal an extra 2d6 necrotic damage and gain a number of temporary hit points equal to the damage dealt. You cannot use this reaction if you already have temporary hit points from this feature or from another source.