Sentinel is divided into three types in Path Of Exile. Stalker Sentinel is available from the moment you pick up the Sentinel Controller, they will follow you to fire their beam attacks at enemies, helping you eliminate them, earn POE Currency or POE Orbs until their time runs out or their energy runs out.

Apex Sentinels function like him, but only attack special or rare enemies. Lastly, Pandemonium Sentinels simultaneously empower enemies with great powers before disappearing. Apex and Pandemonium Sentinel are only available when you power up the slot on the Sentinel controller and not directly like the Stalker Sentinel.

Each Sentinel has an "empowerment" stat that determines the amount of energy it provides to the target. Strengthen the monster's health, defense, etc. However, it also increases the quantity and rarity of the items they drop or the amount of POE Currency. This means that if you use a highly empowered Sentinel, the difficulty of the enemy will be greatly increased, and at the same time, the items obtained will be increased.

The more you use Sentinel, the more it will cost. Higher tier Sentinels usually cost more. If you use up all of Sentinel's charges, he won't charge again. However, you can also use an item called Power Cores to combine two expired Sentinels into a new Sentinel that inherits some of their effects. If you fuse two very powerful Sentinels together it is possible to get a more powerful Sentinel with their powerful abilities, increasing the chance of dropping certain desired rewards.

The above is the introduction to Sentinel. If you need POE Items, you can trade with Path Of Exile service provider POECurrency. Recently, it is running a campaign, when you buy Exalted Orb can give you a lot of extras. Good luck with your game.