The Ur-Ghast can be found on top of the large Dark Towers.
The Ur-Ghast can quickly break armor, so use Naga Scale or Steel Leaf if possible. Higher tiered armor will also suffice. Use a Salamander's Eye to reflect to projectiles.
The Ur-Ghast has 250 health and, similarly to a normal ghast, will shoot fireballs at you. The tears that the Ur-Ghast makes does 3 (1/2 hearts) of damage. It is also not slowed by the Frost Bow. This boss will stalk players and then will attack you, however, due to its size, you can easily see it coming.
The Ur-Ghast will summon Carminite Ghastlings with a lighting strike. The Ghastlings will spawn in ghast traps. This trap is set off by a redstone signal and sucks in nearby Ghasts when powered. To power a trap, the player must slay 2 Carminite Ghastlings near it then activate the block via redstone signal. This will cause a musical pulse, which gets faster per Ghast death. This can draw in the Ur-Ghast to range of a melee weapon.
When the Ur-Ghast reaches low health, it will enter a "tantrum mode,", and will continously spawn minions to attack you. It will also change the weather.
(Please edit at earliest convenience, I merely changed grammar/spelling and did not correct the actual information.)