Currently, there are 8 different special items that you can use within the missions in the game. Although you may only 3 types of special items at a time, and a quantity depending on how much you can afford. These special items are purchased per piece and are much like consumables that are for one-time use only (similar to user spells in other TD games out there).
Special items in the game are purchased using credits, which on the other hand is the premium currency in the game. Credits can be earned by completing the missions, earning them through achievements, by watching video advertisements, or simply by purchasing them via IAPs sold in packages. Special items can provide you some additional area of effect damage in the game, one that freezes enemies in proximity, and some that basically increase the flow of your in-mission resources (etherwatts).
Here are the list of Special Items in the game:
1. Bouncers (50 credits) - A set of three precision bombs that detonate in a small area around the selected target.
2. Etherium Surge (60 credits) - Adds 50 Etherwatts to the reserve, instantly. It may surpass your maximum capacity, until the excess is spent.
3 Replicant Mines (75 credits) - Explosive mines that replicates themselves every few seconds, creating a deadly minefield. Each replicant mine can produce eup to 4 copies.
4. Accumulator (150 credits) - A structure that increases your maximum storage capacity by 50 Etherwatts. If destroyed, it releases stored energy in a deadly blast.
5. Frostbite Warhead (220 credits) - A bomb of liquid Helium-3 that cuts down the temperature in an area to almost absolute zero. All affected enemy units and structures freeze for a few seconds.
6. Proximity Bollard (300 credits) - A defensive structure that remains hidden underground, popping up to electrocute nearby enemies.
7. Mortar Team (500 credits) - Deploy a team of bombing experts! Their long ranged shots can go over barriers. This extra Squad does not count towards your Command Points total.
8. Orbital Strike (1000 credits) - Orbital batteries fire 50 highly explosive missiles at once, devastating enemies in a wide area.
To enlist a Special Item as an active item on the concluding mission, simply drag the icon to the vacant "Equipped" slots tab on the right side of the screen in the Special Items window (accessed through the rocket icon in the main hub UI). To purchase additional quantities for the picked Special Item, simply click on the icon and purchase the quantity you want.