Biological Threat

"Biological Threat" is a special event, available for players level 20 or higher, which originally appeared April 2020, and running for 7 days. It was rereleased in 2021, starting April 9 and running for 14 days, again from April 24 to May 2, 2022, and again from March 29 to April 9, 2023. During the event, you can fight in a special campaign in normal mode (there is no heroic mode for this event). The goal of the event is to unlock areas on the campaign map and win battles in those areas to earn currency that can be spent in the campaign store. This event is the exclusive place to buy some event-themed premium units.

## Campaign

The campaign contains 15 areas increasing in difficulty in a squiggle from the left side to the bottom. The first area, on the left, can be unlocked for free. The rest of the areas must be unlocked by using Healing serum , which can obtained occasionally from victories. If you do not have enough  to unlock an area, you can spend Gold instead at the rate of 50  per missing . You can unlock areas in any order. Once you win a fight in an area, that area will go on cooldown for a length of time. The resources required to unlock an area, as well as that area's cooldown timer, increase as you move deeper into the campaign map. To win a battle, you must either capture the enemy spawn point or destroy all enemy units. If your hero dies or the enemy captures your spawn point, you lose. There's no limit to the number of times you can lose in normal mode battles.

The 2022 version of the event was the same as the 2021 version of the event in most respects, with a few changes: the overall length of the event was shorter, the cooldowns for most levels were increased (despite the overall length being shorter), and the amount of serum from the last two levels was increased. The 2023 version of the event featured no changes from 2022.

Note that in "Fatal error", if you get the serum, you don't get the chest. In all other levels, you will always get the chest.

## Event resources

There are five different resources that can be obtained during the course of the event, two which are used during the event and three that are reward chests.

Healing serum  is a reward for winning battles in the Biological Threat campaign. It is used to unlock areas of the map as well as some items in the event shop. Areas can also be unlocked with gold  instead of .

Medical syringe  is a reward for winning battles on the campaign map. They are used to purchase items from the event shop.

Forgotten chests are a reward event chest for winning battles in the first 5 areas of the campaign map. As event chests, they go to the front of your chest queue and take 1 hour to open. Forgotten chests always contain one type of basic support (randomly selected) and may give: manuals, resources, research trophies and .

Old chests are obtained as a reward event chest for winning battles in the middle 5 areas of the campaign map. As event chests, they go to the front of your chest queue.

Ancient chests are obtained as a reward event chest for winning battles in the last 5 areas of the campaign map. As event chests, they go to the front of your chest queue.

## Event enemy units

The enemy units that you face in battle appear to be relatively normal units at first, but they have a special property -- when killed, they transform into an undead form, which you then have to kill a second time. The enemy units are as follows:

* Infected are the enemy infantry units. They are pretty typical infantry in both forms.
* Host are the enemy stormtroopers. They are regular stormtroopers in the regular form, but in their undead form they are a fast melee unit.
* Zymosis are the enemy heavy infantry unit. They come in groups of two and, like typical heavy infantry, have a high rate of fire but are slow and short range.
* Wandering are the enemy special unit, and function as a sniper unit in both forms.

## Event shop

The items available in the shop are as follows. As is the case for many events, the more powerful items must first be unlocked with resources gained during the event. for this event,  is used to unlock shop items. In the 2021 version, the shop was not available for the first few days of the event, but had the same content as the 2020 version when it was released.

* The purchase limit for healing serum was originally 10 but was increased to 500 on 15 April.

## Detailed map information

The tables below give more detailed information on each map, including the description, troop loadout, and specific details on the levels of the enemy troops.

 **Strategic points** | 1x Machine gun, 2x Mortar

 **Strategic points** | 3x Mortar

 **Strategic points** | 2x Machine gun, 2x Mortar

 **Strategic points** | 4x Machine gun, 2x Mortar

 **Strategic points** | 1x Mortar

 **Strategic points** | 4x Machine gun, 1x Mortar

 **Strategic points** | 4x Mortar

 **Strategic points** | 2x Machine gun, 2x Mortar

 **Strategic points** | 2x Machine gun, 1x Mortar

 **Strategic points** | 2x Mortar

 **Strategic points** | 1x Machine gun, 1x Mortar

 **Strategic points** | 3x Machine gun, 1x Mortar

 **Strategic points** | 3x Machine gun

 **Strategic points** | 3x Machine gun, 1x Mortar

 **Strategic points** | 2x Machine gun, 2x Mortar