Dragonskin halves all damage and also negates Counter and Lethality. It is used by Grima on all difficulties in the Final Chapter and in The Future Past 3. Validar will have this skill starting from Hard Mode and up. All enemy units in Xenologue 22 will have this skill equipped as well.
In Fates, Dragonskin also reduces the effect of critical hits and special skills, as well as preventing all damage dealt outside of battle.
Curiously, Dragonskin gained a new graphic in Fire Emblem Fates. This may be to reflect the dark bluish green of Anankos' scales instead of the pitch black and red of Grima's scales.