Starting a New Game+ will unlock the access to the special boss, the Ur-Dragon!
The Ur-Dragon is an undead dragon version of Grigori. Story has it he wears dead arisen as his armor....
He is located in the deepest chamber of the everfall, the Chamber of Lament.
you can reach there via a special riftstone, located at Starfall Bay in Cassardis. (walk to the far right side of the beach til you reach the end)
There are two types of Ur-Dragon. the OFFLINE and ONLINE.
ONLINE Ur-Dragon is ultimately strong, and has tons of HP since he will receive cumulative damage from players all over the world to be defeated. Whoever deals the final killing blow, will earn neat rewards (most prized ones are likely the legendary weapons) and their name carved into the hall of fame on the stone located near the chamber's entrance
Capcom often held Ur-Dragon hunting events where he will be weaken for a set amount of days.
Each time the online Ur-Dragon is defeated, its next generation will appear with more HP.
OFFLINE Ur-Dragon is alot weaker since he is meant to fight just you instead of the world. You can choose to battle him by simply selecting Offline mode in the options.
To defeat the Ur-Dragon, you have to hit all the glowing parts all over his body until they fell off and reveal rotten flesh underneath.
Everytime you knock a part off, he will drop some valuable materials such as the great dragon scale etc. be sure to pick them up or they will vanish after a while.
Start with the ones on his feet first, then move your way up.
The hardest part will be the tips of its wings... for me personally i just use explosive arrows to take them out.
There are several high grounds on the battlefield, that suggests you could climb up to it and rain fire upon him from a safe distance.
Be careful if you do so, as the Ur-Dragon will fly far away and perch himself on top of a ruin where he will rain fireballs on you instead
You must defeat him within 8 minutes, before he runs away. in such case, simply exit the chamber, go to the inn, rest, and then enter again.
he will appear again with the same amount of HP as he left you last time.
I highly recommend you farm the OFFLINE version of the Ur-Dragon first. Killing him will grant you a legendary weapon, and 20+ wakestones each you can for 30k.
you will also gain 1-2 levels up.
Give it a try and good luck