5. 4 - Níðhöggr
The exact size of Nidhoggr is unknown, but we do know he was large enough to circle one of the three roots of Yggdrasil, which is about a third of the size of Midgard (a.k.a Earth), or maybe a little smaller, but it still leads us to believe this Dragon to be pretty large.
Nidhoggr, also known as Malice Striker or Nidhogg Nager, is a serpentine dragon that dwells below Yggdrasil, spending eternity gnawing on the root to Nifleim. He hopes to one day topple the Life Tree, and destroy all nine realms of the Norse mythology. He is mortal enemies with the great eagle that sits at the top of Yggdrasil, and they send each other insults by the way of Ratatoskr, a squirrel who runs the length of the tree, delivering messages between the two.
He also presides over Náströnd, the Norse version of hell, draining the blood out of corpses and chewing on the dead.
Tales of Nidhoggr’s abilities have been lost in time, but his coming is said to herald Ragnarok, war of the worlds. Other than that, not much is known about this beast.
How to kill:
It is unknown as to how to kill this beast. If Nidhoggr ever attacks you just pray that the gods hear you, for they are the only ones with any chance of beating this dragon in combat, and even then the outcome is uncertain. The best tactic is to avoid Nidhoggr at all costs.
Good luck, by Odin you’ll need it if you ever come across this creature!