The underworld is the underground equivalent of the fortress, in here you can gather souls to upgrade buildings, collect gold and "store" thirst in Adventuromatic. You can go to the underworld by clicking the coin in the bottom right in the Stronghold, or by pressing the Fortress button twice in the menu on the left.

How to unlock underworld?

To unlock the underworld you need to get the Heart of Darkness. It can be mined in a Gem Mine as long as you have at least level 10 Gem Mine and Tower unlocked. After obtaining the heart of darkness, drag it onto your character's avatar.

Heart of Darkness

The Heart of Darkness is the main underground building. The maximum level of the other buildings depends on Heart's level. To access the underworld you need to unlock the Tower and also mine a Heart of Darkness in the gem mine (mine level 10 required).

Heart of Darkness
Level Cost gold Cost souls
1 0 0
2 2 000 616
3 3 000 1 650
4 4 000 4 220
5 5 000 11 000
6 6 000 25 080
7 7 000 45 930
8 8 000 84 150
9 9 000 198 000
10 10 000 439 550
11 11 000 902 850
12 12 000 2 043 300
13 13 000 4 118 400
14 14 000 7 722 000
15 15 000 16 632 000

Underworld Gate

Underworld Gate is a building to lure other players into your Underworld to gather souls. The higher the level of lured hero, the more souls you get for defeating him, so it's important to lure players with the highest level possible. There's a trick to it - you just need to find players with equipment taken off, so they're easier to defeat. You can find them in Hall of Fame or you can ask some guild members to take off equipment for the time of attack.

Underworld Gate can be build after upgrading Heart of Darkness to level 2.

Here is the list of players (without equipment) you can lure into you underworld.

Underworld Gate
Level Cost gold Cost souls Daily limit of heroes
1 500 0 1
2 1 000 55 2
3 1 500 149 3
4 2 000 380 4
5 2 500 990 5
6 3 000 2 250 5
7 3 500 4 820 5
8 4 000 10 090 5
9 4 500 26 730 5
10 5 000 65 930 5
11 5 500 135 400 5
12 6 000 306 500 5
13 6 500 617 750 5
14 7 000 1 158 300 5
15 7 500 2 494 800 5

Soul Extractor

Soul extractor is a building when you can get souls. Those are generated over time and you can collect them like wood or stone in fortress. Souls are very important since it's main resource used to upgrade buildings and units in underworld. Upgrading extractor increases it's production rate and both Heart's and Extractor's storage limit.

Soul Extractor
Level Cost gold Cost souls Production souls per hour Storage limit souls
1 250 0 165 412
2 500 462 231 634
3 750 1 235 330 990
4 1 000 3 165 528 1 716
5 1 250 8 250 825 2 887
6 1 500 18 810 1 254 4 702
7 1 750 40 190 1 914 7 656
8 2 000 84 150 2 805 14 025
9 2 250 198 000 4 125 24 750
10 2 500 439 550 6 105 48 840
11 2 750 902 850 9 405 94 050
12 3 000 2 043 300 14 190 170 280
13 3 250 4 118 400 21 450 343 200
14 3 500 7 722 000 32 175 643 500
15 3 750 16 632 000 49 500 1 188 000

Torture Chamber

Torture chamber increases amount of souls gathered from heroes defeated in underworld. Each level of chamber increases souls' bonus by 10%.

Torture Chamber
Level Cost gold Cost souls Souls bonus
1 660 148 10%
2 1 320 554 20%
3 1 980 1 485 30%
4 2 640 3 800 40%
5 3 300 9 900 50%
6 3 960 22 570 60%
7 4 620 41 340 70%
8 5 280 75 730 80%
9 5 950 178 200 90%
10 6 600 395 600 100%
11 7 260 812 550 110%
12 7 920 1 839 000 120%
13 8 580 3 706 500 130%
14 9 240 6 949 800 140%
15 9 900 14 968 500 150%

Gold Pit

Gold pit is a place that basically generates gold (like Extractor generates souls). Upgrading it increases amount of generated gold and its storage limit. Gold pit can be build after upgrading Heart of Darkness to level 4.

Gold Pit
Level Cost gold Cost souls
1 1 000 12
2 2 000 46
3 3 000 124
4 4 000 317
5 5 000 825
6 6 000 1 880
7 7 000 4 015
8 8 000 8 415
9 9 000 19 800
10 10 000 43 950
11 11 000 90 280
12 12 000 204 300
13 13 000 411 800
14 14 000 772 200
15 15 000 1 663 200
16 10 000 000 162 518 400
--- --- ---
100 10 000 000 162 518 400

Gladiator Trainer

Gladiator Trainer increases damage of critical attacks for main character, companions and pets. It's quite important to upgrade trainer since it helps in dungeons and in pet fights. It can be build after upgrading Heart of Darkness to level 3.

Gladiator Trainer
Level Cost gold Cost souls Crit damage bonus
1 700 148 +11%
2 1 400 554 +22%
3 2 100 1 485 +33%
4 2 800 3 800 +44%
5 3 500 9 900 +55%
6 4 200 22 570 +66%
7 4 900 48 230 +77%
8 5 600 100 950 +88%
9 6 300 237 600 +99%
10 7 000 527 450 +110%
11 7 700 1 083 400 +121%
12 8 400 2 452 000 +132%
13 9 100 4 942 000 +143%
14 9 800 9 266 400 +154%
15 10 500 19 958 000 +165%

Adventuromatic

Adventuromatic is a machine that collects unused thirst everyday at midnight and stores it. Thanks to that you can use that stored thirst so you can fight monsters some time later. There is also a bonus of thirst generated based on adventuromatic level. Beside that, you can "fill" adventuromatic using mushroom .

Adventuromatic
Level Cost gold Cost souls
1 10 000 297
2 20 000 1 105
3 30 000 2 970
4 40 000 5 700
5 50 000 11 880
6 60 000 22 570
7 70 000 41 340
8 80 000 75 730
9 90 000 178 200
10 100 000 395 600
11 110 000 812 550
12 120 000 1 839 000
13 130 000 3 706 500
14 140 000 6 949 800
15 150 000 14 968 500

Goblin Pit

Goblins are weakest units in underworld, but upgrading their pit increases their amount and their stats.

Goblin Pit
Level Cost gold Cost souls Goblins
1 400 0 1 pospolity goblin
2 800 396 2 pospolite gobliny
3 1 200 1 060 3 pospolite gobliny
4 1 600 2 715 4 pospolite gobliny
5 2 000 7 070 5 pospolitych goblinów
6 2 400 16 120 5 wstrętnych goblinów
7 2 800 34 450 5 podstępnych goblinów
8 3 200 63 110 5 przewrotnych goblinów
9 3 600 148 500 5 goblinów w hełmach
10 4 000 329 650 5 ciężkozbrojnych goblinów
11 4 400 677 150 5 dobrze wyposażonych goblinów
12 4 800 1 532 500 5 bardzo groźnych goblinów
13 5 200 3 088 800 5 wyjątkowo groźnych goblinów
14 5 600 5 791 500 5 pieruńsko groźnych goblinów
15 6 000 12 474 000 5 skrajnie groźnych goblinów

Troll Block

Trolls are a little stronger than goblins, but still not that strong as Keeper. Upgraing troll block increases trolls amount and their stats.

Troll Block
Level Cost gold Cost souls Trolls
1 990 165 1 stosunkowo pospolity troll
2 1 980 616 1 troll-siekiernik
3 2 970 1 650 1 troll-siekiernik w hełmie
4 3 960 4 220 1 troll-halabardnik
5 4 950 11 000 1 troll w zacnym hełmie
6 5 940 25 080 1 zbrojny troll
7 6 930 45 930 1 ciężkozbrojny troll
8 7 920 84 150 1 troll z dwoma broniami
9 8 910 198 000 2 posępne trolle
10 9 900 439 550 2 trolle z kilkoma broniami
11 10 890 902 850 2 trolle z pancerzami
12 11 880 2 043 300 2 trolle z wielkimi pancerzami
13 12 870 4 118 400 3 straszliwe trolle
14 13 860 7 722 000 3 straszliwie groźne trolle
15 14 850 16 632 000 4 strasznie grożne trolle

Keeper

Keeper is strongest unit in underworld, since it has best basic stats and best scaling among all units. Upgrading keeper is crucial to face strongest oponents and gather as much souls as possible.

Keeper's chamber can be build after upgrading Heart of Darkness to level 5.

Keeper
Level Cost gold Cost souls
1 1 500 198
2 3 000 739
3 4 500 1 980
4 6 000 5 065
5 7 500 13 200
6 9 000 25 080
7 10 500 45 930
8 12 000 84 150
9 13 500 198 800
10 15 000 439 550
11 16 500 902 850
12 18 000 2 043 300
13 19 500 4 118 400
14 21 000 7 722 000
15 22 500 16 632 000