How the Underhold works
A pixel dungeon 2222 floors deep. Every mint sends a hero one floor down. Bosses fall, relics pay, and the torch decides when it all ends.
Overview
- 2222 heroes, one per floor, on Robinhood Chain.
- 11 tiers of 202 floors, from the Cellar to the Heart. Each tier has its own look and its own boss.
- Every mint pays the players: 40% to the tier's vault, 15% to the heroes already below (the Tavern), 15% to the Heart.
- The torch burns while people mint. If it goes out, the last hero down takes the Heart.
- Fully on-chain: the contract draws every image and writes every trait. No IPFS, no servers.
- Raids open after the mint: parties of heroes go back into the depths for loot and trophies.
The descent
Your mint number is your floor: the first hero lands on floor 1, the last on floor 2222. Floors fill in order, so the deeper the mint, the later it happened.
| Tier | Floors | Tier | Floors |
|---|---|---|---|
| 1 · Cellar | 1–202 | 7 · Forge | 1213–1414 |
| 2 · Sewers | 203–404 | 8 · Crystal Mines | 1415–1616 |
| 3 · Catacombs | 405–606 | 9 · Bone Pit | 1617–1818 |
| 4 · Fungal Caves | 607–808 | 10 · Abyss | 1819–2020 |
| 5 · Drowned Halls | 809–1010 | 11 · Heart | 2021–2222 |
| 6 · Frost Crypt | 1011–1212 |
Deeper heroes carry better gear: rusty iron near the top, gold in the middle tiers, obsidian and glowing weapons near the Heart.
Price and limits
The price is fixed in ETH and rises with depth: 0.00004 ETH on floor 1, ×1.8 per tier, a little more on every floor (about 0.29% per floor). There are no steps between tiers.
| Floor | Price | Floor | Price |
|---|---|---|---|
| 1 | 0.00004 Ξ | 1213 | 0.00136 Ξ |
| 203 | 0.000072 Ξ | 1617 | 0.0044 Ξ |
| 405 | 0.00013 Ξ | 2021 | 0.0143 Ξ |
| 809 | 0.00042 Ξ | 2222 | 0.0256 Ξ |
- Up to 5 heroes per transaction.
- Up to 20 heroes per wallet in each tier. The count starts again in the next tier.
- If someone mints right before you, your floor is a bit deeper and costs a bit more. The site pays 10% over the quote to cover that; anything unused comes back in the same transaction.
- On the last floors you can mint only as many as are left.
Rooms and traits
Every hero gets a Class, a Room (what it found on its floor) and a Background. Odds are fixed in the contract.
Room
| Room | Chance | What it means |
|---|---|---|
| Empty | 40% | just the floor and a torch |
| Monster | 30% | the hero slew whatever lived here |
| Chest | 25% | loot, shown on the picture |
| Shrine | 2.5% | the hero is blessed; counts in raids |
| Relic | 2.5% | pays: relic holders share half of their tier's vault |
Class
10 classes, 10% each: Knight, Rogue, Mage, Cleric, Ranger, Barbarian, Necromancer, Monk, Dwarf, Bard. Each class has its own heads, armour and weapons, so heroes of one class still look different.
Background
| Background | Chance | Background | Chance |
|---|---|---|---|
| Plain | 34.5% | Throne | 4.6% |
| Library | 8.0% | Fall | 4.6% |
| Armory | 8.0% | Statue | 4.6% |
| Prison | 8.0% | Vault | 2.9% |
| Alchemy | 6.9% | Portal | 2.3% |
| Crypt | 6.9% | Rift | 1.7% |
| Mine | 6.9% |
Two more traits appear during the mint: Bossslayer (the hero that slew its tier's boss) and Last Light (the last hero down if the torch goes out).
Where the ETH goes
| Share | Goes to | Paid when |
|---|---|---|
| 40% | the vault of the tier being minted | when that tier's boss falls |
| 15% | the Tavern: every hero already minted | immediately, claim any time |
| 15% | the Heart | at floor 2222, or when the torch goes out |
| 30% | the team / raids | — |
Vaults, the Tavern and the Heart are held by the contract and paid out only by its rules. The team cannot touch them.
The Tavern
15% of every mint is split equally among all heroes minted before it. The earlier a hero, the more mints it shares in. Tavern earnings belong to the hero: if you sell it, unclaimed earnings go with it.
Bosses and relics
When a tier's last floor is minted and revealed, its boss falls. One hero of that tier becomes the Bossslayer. The pick is random but weighted by depth: the k-th floor of the tier has weight k, so the 202nd floor is 202 times likelier than the 1st.
- Bossslayer: half of the tier's vault (all of it if the tier has no relics).
- Relic holders of that tier: share the other half equally.
- The last boss (tier 11) also takes the Heart when floor 2222 is reached.
The torch
The torch is lit when the mint starts and burns for up to 60 minutes. Every hero minted adds 2 minutes, up to the 60-minute cap. While people mint faster than about 30 heroes an hour, it never goes out.
If it reaches zero, the mint ends for good and the last hero down takes the Heart plus the vault of the tier it is in. A slow mint is not a dead mint: every hero could be the last one.
How the mint ends
| Ending | What happens |
|---|---|
| Floor 2222 is minted | the tier 11 boss falls; its slayer takes the vault share and the Heart |
| The torch goes out | the last hero takes the Heart and the open tier's vault; the supply stays at the number minted |
Anyone can end the mint once the torch is out: a late mint attempt ends it and refunds the sender, or call extinguish().
Claiming
Everything a hero earns (Tavern, boss, relic, Heart) sits in the contract under that hero's number. Whoever holds the hero claims it from Your heroes on the mint page, or by calling claim([ids]). There is no deadline.
Reveal and randomness
A hero's traits come from a seed: the hash of the block it was minted in, read only after that block is sealed. Nobody, including the minter, can know the seed inside the minting transaction.
- Seeds are revealed by the next mints and by a keeper; anyone can call
settle(). - Until then the hero shows as Descending. The picture appears a few seconds later, and marketplaces are told to refresh (EIP-4906).
- If a seed is not revealed within ~256 blocks, it falls back to entropy fed by every mint, so the moment someone settles cannot change the outcome.
On-chain art
Every image is drawn by the contracts from the hero's floor and seed: walls and floor, the biome's props or a themed background, what the hero found, the hero itself, and torchlight in four animation frames. The result is an animated SVG returned by tokenURI, together with every trait. Nothing is stored off-chain, and the art can be rebuilt from the chain forever.
Raids AFTER THE MINT
Party
Send 1 to 4 of your heroes on a raid. Each hero can be in one raid at a time. Every class has a role:
| Role | Classes |
|---|---|
| Tank | Knight, Dwarf |
| Damage | Barbarian, Rogue, Ranger |
| Magic | Mage, Necromancer |
| Support | Cleric, Monk, Bard |
Power
| Source | Power |
|---|---|
| Tier of the hero's floor (1–11) | = the tier number |
| Relic room | +3 |
| Shrine room | +2 |
| Chest or Monster room | +1 |
| Bossslayer | +3 |
| Every 5 raids won | +1 (up to +3) |
Backgrounds are cosmetic and give no power.
The raid
Pick a tier T from 1 to 11 and pay the entry fee for each hero. The raid takes one hour; then a single roll decides it.
| Party | Power | Raid tier | Chance |
|---|---|---|---|
| 1 Knight of tier 3, empty room | 3.45 | 1 | 37% |
| 4 heroes of tier 6, Monster rooms, one role | 32.2 | 6 | 47% |
| 4 heroes of tier 6, Monster rooms, four roles | 44.8 | 6 | 55% |
| the same party | 44.8 | 11 | 40% |
| 4 heroes of tier 11 with Relic and Bossslayer, four roles | 81.6 | 11 | 55% |
Rewards
The first part returns the entry with a premium for the risk; the second is a bonus from the raid pool that grows with depth. A single raid pays at most 1% of the pool.
| Trophy | Earned by |
|---|---|
| Cleared · tier N | first win in a tier: a pennant of that tier's colour on the picture |
| Veteran | 10 raids won |
| Heartbreaker | a win in tier 11 |
| Raid Bossslayer | killing a boss in a boss raid |
Boss raids
Every 6 hours one tier's boss rises again, in turn. The first party of 4 to kill it (double difficulty) takes 5% of the raid pool and the Raid Bossslayer trait. The window lasts until the boss is killed or 6 hours pass.
Failure
- Entry fees stay in the raid pool.
- Each hero in the party is wounded for 2 hours: it cannot raid and wears a bandage on its picture. Wounded heroes can be sold and transferred.
- Heroes are never burned and never die.
The raid pool
The raid pool is funded by entry fees and by the team, which adds to it at its own discretion. Once ETH is in the raid pool it can only be paid out by the raid contract.
Team and royalties
- 30% of every mint goes to the team / raids.
- Royalty on secondary sales: 5% to the team (EIP-2981).
- The owner can replace the art renderer and the randomness source until
freeze()is called; freezing is permanent. It is planned right after raids launch, so raid trophies can be added to the art first. - The owner cannot touch tier vaults, the Tavern, the Heart or the raid pool.
Contracts
Robinhood Chain (chain ID 4663).
| Contract | Address |
|---|---|
| Underhold (mint, payouts, NFT) | 0x2d35B2e287E8179D4cE25159F112452746ceefAD |
| UnderholdRenderer (art and metadata) | 0x1f371a44BF9F30029a22706155E27bF39CAAB1d4 |
FAQ
My hero shows "Descending". Is something wrong?
No. Its seed is revealed in the next block; the picture and traits follow within seconds. On OpenSea, use "Refresh metadata" if it lags.
Can I pick my floor?
No. You get the next free floor. Traits come from a seed nobody can know in advance.
Do I lose my earnings if I sell?
Unclaimed earnings belong to the hero, so they go to the buyer. Claim before you sell.
What if the torch goes out while my transaction is pending?
The transaction does not fail: it ends the mint and refunds you.
Is there a whitelist or a free mint?
No. Everyone mints from floor 1 up at the same price curve.