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This a popular late-game build for Zerg in which a Zerg player with 3 or more bases builds a steady stream of Ultralisks and Zerglings with both Metabolic Boost and Adrenal Glands, known as "Cracklings". In addition, upon reaching Ultralisks a Zerg player has access to units to lessen the effectiveness of Zealots and Sentries' Force Fields through Infestors' Fungal Growth and Ultralisks breaking Force Fields due to being a massive unit.
This build focuses on the continual output of Ultralisks and Cracklings, with support from other units as necessary. It does extremely well against any kind of Robotics Facility-based force, which many Protoss players tend to favor in the mid- to late-game.
Ideally, you want to open with a build that allows for a fast expand while minimizing gas use. Sheth's Speedling Expand is a good example of such a build. Additionally, 3 Hatch Muta/Ling (vs. Protoss) can lead into this, though severely delayed because of the gas cost of Mutalisks.
 Build Notes
Producing these units at a steady pace is difficult with less than 3 bases (1 high-yield) or 4 bases (no high-yield). Ultralisks are extremely expensive, at 300 minerals and 200 gas each. Zerglings are cheap, but their upgrades aren't. Adrenal Glands is a 200 mineral, 200 gas upgrade, which is fairly steep. In addition, most players tend to get Chitinous Plating for their Ultralisks to increase their durability; this is a 150 mineral, 150 gas expense.
Naturally you want to be continuing to upgrade both melee weapons and carapace from your Evolution Chambers throughout the game. Many players like to start upgrading far in advance of Hive tech for several reasons.
- Most Protoss players get +1 Weapons, which allows Zealots to two-shot Zerglings unless they have +1 Carapace to even the field.
- By completing these upgrades ahead of your Ultralisk Cavern, it allows you to spawn Ultralisks with +3/+3 right away, and they can be +3/+5 even faster. You'll also be able to complement them with +3/+3 Zerglings.
- Ultralisks with +3 damage inflict 47 damage per hit against armored units and buildings.
- Zerglings with Adrenal glands and +3 damage inflict 8 damage per hit. This means they retain full damage against Immortals, but their quick cooldown allows them to quickly wear down Immortal shields. In addition, they do nearly as much DPS as a fully upgraded Zealot (13.62 and 18.33, respectively).
- Make it a priority to spread the creep throughout the game. The speed bonus conferred by creep is invaluable, as Ultralisks suffer a large speed loss off the creep and Zerglings can create surrounds more easily on it.
- Create control groups of Zerglings and Ultralisks separately. Focus your Zerglings on the opponent's Immortals and Zealots while your Ultralisks focus on Colossi and anything else. Immortals deal well with Ultralisks due to the Hardened Shield ability and their bonus damage vs. Armored, but Zerglings are unaffected by the Hardened Shield since they do <10 damage. Meanwhile, Ultralisks deal bonus damage to Colossi and Stalkers, and are extremely robust so they can take on multiple opponents.
Hopefully you will not need to do much adaptation, because you only opted for this end-game mix after noticing the opponent's Robo army. Here are some notes and twists that might increase the build's flexibility.
- In the mid-game, if the Protoss player has shown Colossi then most Zerg players will build a Spire for Corruptors or Mutalisks. Do not ignore this structure in the late-game; a few Corruptors can really bolster your force by focusing down Colossi and using their Corruption ability on infantry.
- You need to build an Infestation Pit to upgrade to Hive tech. If the Protoss player has switched to a Gateway-heavy mix then Infestors can come in handy with their Fungal Growth ability. Fungal Growth is especially helpful in dealing with the high number of Zealots many players build in reaction to such high numbers of Zerglings. In addition, if you research Neural Parasite then rather than killing Colossi you can mind-control them.
 Pro features
Open maps that allow you to readily establish concaves and surrounds with your units, and make it difficult for the Protoss player to abuse Force Fields.
 Con features
Tight maps with lots of chokes and high-ground that Colossi can abuse. In addition, any map with a long ground rush distance which will increase the time it takes you to reinforce your army.
IdrA vs. HuK, Game 3, Day9s King of the Beta Tournament. IdrA opts for a late-game Ultra/Crack composition and breaks HuK's Robo mix, aided by the fact that HuK neglected to research Extended Thermal Lances for his Colossi.