Dragons are tough, fire-breathing monsters with a large amount of Life that matches its breath attack. It spawns midway in the game, usually in the volcanic terrain or in the Corrupted Lands. One of the few enemies who rarely drops save crystals.
On higher Difficulties, Dragons are able to almost oneshot you with even a Tanky oriented Build/Skill Order. On Inhumane Odyssey they can even oneshot a full Vitality Knight during midgame. Even if you reduce incoming damage through Spells such as Phalanx, you still die most likely with one hit. They also have no Cooldown or casting time (Meditation) on their attack. Try avoiding them completly, if you haven't saved recently/cannot oneshot them.
An option against higher difficulty dragons is if you stay on range, since their Fire-attack is melee.