Fafnir is the first boss that Volgarr must face after completing the tower. If Volgar arrives at Fafnir's room via the Valkrye levels then there is another boss after Fafnir. If you have arrived via regular levels, he is the final boss you must fight to beat the game. Beating Fafnir will present Ending C.
Fafnir lives in a large room with spikes on each side. His chamber has a shield, helmet, and flaming sword right outside of it. His room also has a small house in the corner that Volgar can go into to avoid most of Fafnir's attacks.
Fafnir will transform from a man into a large dragon holding a glowing ball when Volgarr enters the room, and starts his attacks of large flame balls thrown at Volgarr. This is the first attack in his pattern and is termed the "1-2-3" as there are 3 fireballs.
Fafnir's second attack is the lightning that covers the floor for a second. Volgar can avoid this by single jumping while standing anywhere except inside the house.
Fafnir's third attack is the "1-2-3" , Volgar can avoid this by staying in the house
Fafnir's fourth attack is the "bombardment" which can be avoided by staying in the house.
Fafnir's fifth attack is the "1-2-3" which is avoided similarly
Fafnir's sixth attack is the "open worlds" move where the dragon flies to the bottom middle of the room and casts a spell that can change the room into one with more spikes. The open world boundary will hurt Volgarr once the spell is cast which takes a few seconds. Casting this spell is when Fafnir is most vulnerable but you only have time for 3 or 4 spears before you have to roll back into the house.
Fafnir will then start over with his first attack, the "1-2-3." Fafnir will do these 6 attacks in this order every time and can be defeated most easily by first spearing the left wall in several places, and then climbing to the top spear and throwing super spears at Fafnir until his 3rd attack of the first 1-2-3 sequence. Then Volgarr should avoid the lightning attack (see 2nd attack above) and then stay in the house until the 6th attack which is when Volgarr can get the rest of his hits in. Then Volgar can go back into the house and wait for the lightning attack, at which time Volgarr needs to be outside the house and ready to jump to avoid the attacks. Then go back into the house and await the open worlds attack which is when Fafnir can be hit.
After defeating Fafnir, Volgar climbs down a rope and an ending is presented with a story based on which ending Volgarr has earned. Ending C (the easiest ending) shows a monologue from Fafnir about how he set up his empire and Volgarr arrived, battled through his worlds defeating his minions, and basically messed everything up. Volgarr is called a "jerk" in this scenario, and Fafnir says Volgarr needs to stay away from his gold.
Then the end credits occur which name each creature found throughout the game. Surprisingly, the names you may have used such Poking Dingus, the Sliding Dingus, and the Jumping Dingus are called Myrmor defenders or some such.