How to "level quickly"? Where to level?

Required Level	Underground
20	Relics of Arid Moonlight
30	Lost Mines
50	Crystal Valley
70	The Shrine of Palmir Plateau
105	The Sanctuary
120	Temple of the Ancients
130	Temple of the Lost Souls
140	Temple of Exile
150	Cube
150+	Circus, Moonlight, Lost Mines, Crystal Valley and Palmir Plateau Stage 1 and 2
160+	Circus, Moonlight, Lost Mines, Crystal Valley and Palmir Plateau Stage 3 and 4
160+	Island of the Forgotten Gods. To reach the Island, you have to be on the board of a ship called “Sheppard”, before doing that you have to buy a ticket "Sheppard Boarding Pass", which you can buy at Black Marketer. Ship leaves to Island every 30minutes, хх:00 and хх:30.
160	Remains of the Ancients. To enter Remains of the Ancients, you have to finish a questline "Truth of Witch", which u can start at City of Ruins after reaching level 160.
175+	Devildom. There are no requirements to enter Devildom, you just have to be level 175.
200+	Dark laboratory 1st floor. New unique dungeon, much more complicated than Devildom.
205+	Dark laboratory 2nd floor. New unique dungeon, much more complicated than Devildom.
210+	Dark laboratory 3rd floor. New unique dungeon, much more complicated than Devildom.

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What items to use? Where/How to get endgame gear?

Accessories and Belt

Till level 180, most classes use the "Bear Skin Belt", which increases skills by +1, and it drops in Circus.

After reaching level 180, it is optimal to make Yushiva's Belt. 

Recipe for Combination Window:

• 50000 Extreme Power
• 50 Yushiva's Essence
• 500 White Dragon's Soul Bead
• 300 Black Dragon's Soul Bead
• 200 Sand Dragon's Soul Bead
• 500 Refined Crystal of Devildom
• Any "Bear Skin Belt"

Most of the required items drop in Devildom.

Rings after level 170

Soul of Mortis and Ring of Horadric, which drop on Island of the Forgotten Gods. They can have up to 3 stats, including crit. power.

Earrings and Necklaces for level 160 drop in Circus. On 175+ you should switch to Yushiva's accessories, necklace increases damage against certain types of creatures, also humans, earrings on the opposite, decrease the damage you take from certain types.

Weapons

Level 160 weapons you can get by killing bosses in Circus. Level 170 weapons drop from bosses on Island of the Forgotten Gods, and also you can exchange 20 "Sign of the Forgotten Gods" at the npc "Luciad Society Ruth", with a chance to get a 170 weapon or a ring.

Set(Helmet, Armor, Gloves, Boots)

Medusa's helmet drops in the underground dungeon of Relics of Arid Moonlight, you can access it by talking to the NPC outside the dungeon. It drop's from a box that you open after completeing the dungeon. On stage 3 the chance to get a helmet is 100%, however, on stage 4 it drops 2 helmets.

Armor, gloves and boots of Parallel World - these drop from bags which you obtain by killing the bosses in the Parallel World. You can improve these items to Devildom grade ( Nusylite, Fluminis, Inferno, Reviac).

Improving the Sets

To improve the set you will need a piece of armor, boots, gloves (skadi, daman, mach or nemon), also you will need  Distorted Space's Armor Upgrade (blue sphere, which drops from Island), and the last is random items 160 from parallel world.

Crafting Recipe: 

The item you want to improve(main Slot)+Distorted Space's Armor Upgrade and 160 item same type(gloves to improve gloves etc.)

In the end you get an Enhanced armor piece with a star. 

To continue upgrading, you will need 50 Refined Crystal's of Devildom and the armor piece equipped, to start upgrading the item, go to Katan Blacksmith.