Sentinels

Sentinels are robotic companions that follow players around and assist in various ways depending on their set Precepts. It is possible to program sentinels with precepts, or skill mods, that can turn a sentinel into an attack drone or a support drone. It can also have many other mods which, for the most part, function similarly to warframe mods.

Sentinels, like weapons and warframes, level up with affinity. They have health, shields, armor, a weapon of their own (they cannot use it until the respective precept is installed), energy reserve and lots of utility. They can be targeted separately from a player's frame and, upon death, do not revive for the rest of the mission, unless the user dies and spends a Revive point. Enemies tend to target a frame first, but will often target the sentinel instead.

There are three kinds of sentinels, each taking one of ten sentinel slots (which are different from warframe and weapon slots). All Sentinels will come with two mods specific to their type installed at creation. Sentinels have 4  polarity slots, an Orokin Reactor upgrade slot, and can be Polarized with a Forma.

Cosmetics can be applied to sentinels, but currently serve no functional purpose.

Sentinel Mods
There are exclusive Sentinel Mods for each Sentinel. Only the Shade can use the mods Ghost and Revenge, the Wyrm has Crowd Dispersion and Warrior, and so on. There are 10 precept mods slots and 9 possible mods for those slots.

Sentinel AI
Sentinel's use their precepts in priority order, depending on which slots you install the precepts into. This is extremely important to the Sentinel's functionality but not at all apparent in the Sentinel Mod UI. Sentinels will use precepts in a prioritized order as follows [from highest to lowest priority]: This means that if you install, for example, Swift Death, into the top left precept slot, it will kill all enemies and only use Vaporize if it has the opportunity to in between firing - it will not stop to use Vaporize or Guardian if it is shooting already.
 * 1) Top Left Precept Slot
 * 2) Top Right Precept Slot
 * 3) Bottom Left Precept Slot
 * 4) Bottom Right Precept Slot

In the case of Dethcube [or Wyrm/Shade], It is thus recommended to slot Guardian in the Priority 1 slot, and Swift Deth [or Warrior for Wyrm/Revenge for Shade] into the Priority 4. This means it will stop shooting things to regenerate your shields, which is more important to the Tenno's survival.

Acquisition
Dethcube, Shade and Wyrm can be bought for 75 and the blueprint for  100,000.