In view of the strong electron-donating nature of H(-) and extensive vacancy formation in metals by hydrogen insertion, a series of LnH2+x (Ln = La, Ce, or Y) compounds with fluorite-type structures were verified to be the first hydride-based electride, where itinerant electrons populating the cage are surrounded by H(-) anions. The electron transfer into the cage probably originates from Ln-cage covalent interaction. To the best of our knowledge, anion-rich electrides are extremely rare, and a key requirement for their formation is that the cage site is not occupied by lone pair electrons of the adjacent ions. In the case of LnH2, the cage site is surrounded by eight H(-) anions with isotopic electronic character caused by the lack of mixing of H p-orbital character. Notably, Ru-loaded LnH2+x electride powders synthesized by hydrogen embrittlement (Ln = La or Ce) were found to work as efficient catalysts for ammonia synthesis at ambient pressure, without showing serious signs of hydrogen poisoning. There are several possible origins of the observed high catalytic activity in the hydride promotors: the small work function of LnH2+x derived from the covalent interaction between Ln cation and the H(-) σ donor, and the formation of Ln nitride during catalytic reaction.