Underworld is fantastic. This is long, so I apologize, but I hope it will be of help to you.
Underworld
TLDR: damage the boss to stun her, pick a mix of creatures that work reasonably together when you can. Kill tombstones when they are up, and creature crates when you can, but maintaining your health is more important than either.
Intro:
Underworld is a survival mode that tests the strength and depth of your roster. You complete a series of stages (collectively known as a chamber) and try to reach the end before losing all of your creatures.
The main reward for this mode is crafting components for idols, both from chests and urns. There are quite a few mechanics at work, but we hope this guide helps you get started.
Basic Features and Mechanics:
The goal of each individual stage is to avoid taking damage and delay Hamo from capturing your creatures for as long as possible.
When the mode begins, you will be given the choice between two units of each color to start your assault on the chamber. You will be unable to change these unless a unit is captured.
Hamo (Hel’s Minion): Hamo does not attack directly. On the turn after her hold counter reaches zero (the hourglass will disappear) she will imprison one of your creatures. Her current HP is shown in a circle next to the hourglass. Reducing her HP to zero will reset her HP (you can’t damage her any more that turn [thanks ArtNJ]) and add one turn to her hold timer. Hamo’s HP and hold timer carry over between stages, so damaging or stunning her at the end of a stage is still helpful. The amount of damage required to stun her increases as you progress through the chamber.
Your HP: If your HP drops to zero you will lose 3-4 creatures immediately, so your number one priority is to avoid taking damage when possible. Like Hamo’s health, your health carries over between stages.
Turn Counter: When the turn counter reaches 0, the stage ends. Note that Hamo can still capture a creature after the end of the least turn.
Garm: Stages featuring Garm are completed by reducing his health to zero. He attacks in 2 columns with his strength every turn and this stays constant throughout the fight. There is still a turn counter; if the counter reaches 0 before he is dead, he attacks immediately with his remaining hp as his strength. This can cause you to lose a whole lot of health if you aren’t careful or get caught with a bunch of creatures with long hold times.
Unit capture: When a unit is imprisoned, you will choose between two options for a replacement, unless you have less than two remaining of that color. They are not permanently lost, but can be regained rom creature crates.
Stage Features:
Minions and Constructs: The other enemies are the major threats to your health. Watch out for ballista and other enemies spawning behind urns. In Chamber 3, Undead and Tentacles appear.
Bombs: Use these to your advantage to damage everything. If a stage has directional bombs, it is often a good idea to hit them early.
Urns: Destroy them if you can, as they contain mold components.
The back wall: Like Treasure Caves, you can damage the back wall and drop stones from the ceiling to damage trash minions, urns, or anything else on the board, including Hamo.
Stage Completion and Rewards:
After you complete most stages, you will get to activate a spinner which can provide a number of goodies (see below) but will usually make you question how probability works. You can buy more spins (25 gems), but it is not recommended.
Some stages award you with a material chest, a creature crate, and a tombstone for the next level. At these points you will also find a shop where you can purchase a health potion or a creature crate.
If you finish the chamber, you will receive a large number of idol component components. Note that in the third chamber, you are given three choices to pick from, so it is good to know which type you might need before you finish. In chambers 4 and 8, you will receive 35 components and a rare idol.
The third chest of chamber 6 yields 50 components. Not a bad chest to farm for a few runs if you are running short
Helpful Items:
Tombstones: These can be earned from the spinner. You also get one each time you reach a chest. Two tombstones spawn at the start of the stage and hold for 3 turns before they disappear. If you kill them both before they disappear, you will generate a Creature Crate.
Creature Crates: These can be won from spins, from destroying the pair of tombstones, or purchased from the shop (starting at 75 gems). Unlike tombstones, these do not disappear after the round ends and their health carries over to the next stage. You can also have two creature crates up at once, so always kill tombstones first before crates. The color will be predetermined and you will pick between 2 units, provided you are missing more than one of that color.
Hourglass: From the spinner. Resets Hamo to full hold.
Health Potion: From the spinner or purchased from the shop (cost starts at 50 and increases). Resets your HP to full.
Underworld Ticket: Resets your progress to the beginning of the current chamber and all your creatures are rescued.
Strategy Tips:
The chamber gets more difficult as you progress. Save some of your best creatures for the end, but don’t use all your bad creatures at once.
Try to have 1-2 units that launch immediately or after 1 turn on your team at all times. If you get stuck with a full team of long hold units, it can make life very difficult.
DoTs are very effective at dealing damage to Hamo while you focus other threats. Units like Fire Strider can be incredibly useful in Underworld for this reason. Use this opportunity to learn more about units you probably don’t use very often.
Think about synergy of units when picking replacements. Flame Wyrm is an awesome pick if you have that Fire Strider, Valkyrie, or another fire-based unit on the field. Not so much if you have no fire creatures in play.
If your health is low and you could wipe at any moment, it might not be the best time to choose one of your star units as a replacement. (Thanks ArtNJ)
If Hamo is about to steal a creature, check the color of the skull over her head. That is the color she is about to steal. If you combine a creature of that color, it will then attack when she steals it.
Near the end if you start running out of a few colors, you can have 2-3 good creatures left and easily make matches all over the board, so don’t despair if you look like you’re going to fail near the end. Be warned that once you are down to 2 creatures, Hamo will attack with her current strength instead of stealing a creature.
Protect your HP: you will lose 3 creatures if your health is reduced to zero (the colors are shown by the skulls at the bottom of the screen), whereas you only lose 1 creature if Hamo imprisons one at the end of her hold timer. Letting a big hit get through to finish a tombstone or stun Hamo is a bad move.
If a creature gets imprisoned, any combined units of that type will attack immediately. You can use this to your advantage at times.
Hamo always moves one space left or right each turn, and telegraphs the direction with the box underneath her with the exception of when you stun her, so use that plus knowledge of your creatures hold time and attacks to damage her.
It is worth repeating that hits against the back wall can drop stones on the field. These can be a huge lifesaver as a last resort, especially on stages with Garm
