Hollow Zero — Zenless Zone Zero

Hollow Zero, also known as Limbo, is the largest known Hollow in Zenless Zone Zero. It is also a major game system accessed in Scott Outpost. Per the New Eridu Settlers Guide Vol. 5: "It's hard to talk about Hollows without mentioning Hollow Zero, the beginning and the end of a great many things. Everyone knows how dangerous it is, but that doesn't stop people from being curious. If anything, it's why people want to explore it." Hollow Zero consists of several game modes with differing gameplay styles: Withered Domain, Lost Void, and Arpeggio Fault.

License Level is Hollow Zero's account progression system: clearing Withered Domain or Lost Void stages awards Investigation Points, and each License level requires 1500 Investigation Points. Lost Void and Withered Domain run on two SEPARATE License tracks — the current caps are Lv.200 for Lost Void and Lv.90 for Withered Domain. There's a weekly cap on how many points you can earn, and that cap rises as you level up your License. Each track's level rewards include its own namecard + avatar, plus 5 copies of a track-exclusive Resonium (Withered Domain: Winding Roses namecard + Fragrant Infusion avatar + 5 Booressure; Lost Void: Puppeteer's Letter namecard + Final Dose avatar + 5 Brawlerboo). The community wiki names 5 unlock milestones for tougher zones/difficulties, in order: clear Interior-difficulty or higher → clear Heartland-difficulty or higher → clear Core-difficulty or higher → complete Withering Garden: Core → complete Withering Garden: Seedbed — but it does NOT attach a specific License level number to each milestone, so the level numbers are marked not disclosed; only the sourced difficulty conditions are listed.

Withered Domain gameplay consists of the player controlling Eous around the Monitor Array, much in the same vein as HDD Commissions, with combat taking place in instanced arenas. The player will choose a stage and a single starter Agent, then explore a procedurally generated Monitor Array, encountering various Hollow Events that can provide differing effects. In this mode, the player must also manage Pressure and Corruptions. Withered Domain has a 6-zone structure that unlocks progressively: Qualification Assessment (2 stages: Edge, Frontline) → Old Capital Metro (5 stages: Edge, Frontline, Interior, Heartland, Core) → Construction Ruins (4 stages: Frontline, Interior, Heartland, Core) → Abandoned Skyscraper (3 stages: Interior, Heartland, Core) → Withering Garden (1 stage, with modifiers to raise difficulty and rewards) → Inferno Reap (the final zone, a fight against an undefeatable version of Nineveh, scored by total damage dealt). The Resonia kit collected in Withering Garden can be kept and carried into the Inferno Reap fight.

Lost Void gameplay does not feature Monitor Array content, instead focusing on travelling between procedurally generated instanced-based events and combat encounters. Per the community wiki change history, Lost Void was released in Version 1.4, the same update in which Hollow Zero was restructured — with the original Version 1.0 "Array Mode" content being moved into Withered Domain. Lost Void has 6 modes: Task Force Investigation, Perpetrator Battle, Battlefront Purge, Shadow Operation, Special Operation, and Operation Matrix — each mode swaps the pre-run buff selection and changes which Agents are available. Task Force Investigation pits you against Miasma Priest or Wandering Hunter as the final boss, with rewards and featured Agents refreshing periodically, so it's worth clearing at least once a week. Operation Matrix is the newest mode (added in Version 2.6) and the first to use an Auxiliary Agent — one extra support Agent added on top of the usual 3-Agent, 1-Bangboo squad.

Arpeggio Fault gameplay is purely focused on Monitor Array gameplay, with all combat taking place within the Array itself. In this mode, instead of managing Pressure and Corruptions, the player must instead manage the "HP" and "ATK" stats of not only their accompanying Agent(s), but also themselves. Per the community wiki change history, "The Mystery of Arpeggio Fault" content was released in Version 1.3.

Version 2.6 brought 3 major changes to Hollow Zero. (1) The UI was reorganized: game modes are now split into 2 main groups — Periodic and Standard. (2) The Operation Matrix mode launched in Lost Void, the first to use an Auxiliary Agent — one extra support Agent added on top of the usual 3-Agent, 1-Bangboo squad (see the Lost Void block). (3) The Deep Cleaning Service Card was added: using this item instantly clears a stage and collects its rewards for Withered Domain, The Mystery of Arpeggio Fault, and certain other stages — no manual replay needed.

While exploring Hollow Zero, Agents may collect an assortment of Resonia. Resonia give your team various combat buffs or an exploration buffs, which can be viewed in the Resonium Inventory on the left. Resonia can also be bought in Bangboo Shops in exchange for Gear Coins gathered during exploration, or picked up in certain mission-specific events. Per the community wiki (Japanese edition), if the squad includes an Agent with Bond at least "Best Friend" rank (Rank 3), stepping on the central supply-point tile after defeating a floor boss has a chance to trigger a special event; clearing it grants that Agent's own Signature Resonium and unlocks one Mindscape-style effect that only applies inside Hollow Zero.

Obtain Shadow Crystal Data from first clear rewards and regular rewards to complete the weekly and limited-time Bounty Commissions and obtain generous item rewards. You can complete 4 Bounty Commissions each week, with the Commissions divided into 4 difficulty tiers. As your Inter-Knot level increases, higher-tier Bounty Commissions will be unlocked during the next update, and completing them will reward you with even more generous rewards. Bounty Commissions will refresh every Monday at 04:00. Completing Bounty Commissions also rewards Z-Merit, which can be exchanged for valuable items such as Master Tapes and Boopons at Casper's Shop (NPC Casper, outside the Hollow Zero tent). Casper's stock resets on the 1st of every month, so spend down what you want before the month rolls over.

Speaking to Ray at Outpost Interior will open Ray's Research Log, which acts as both an informational database and a reward system for discovering things and completing achievements within Hollow Zero. Here, the player can check their acquired Resonia, Corruptions, Special Areas, Collectibles, and Primers, and earn rewards based on the amount of things they have discovered — these rewards are mostly Polychrome. The player may also review various achievements they have completed within Hollow Zero, and collect rewards for completing achievements.