“You're not supposed to be in here! You're going to ruin the surprise!”This article contains spoilers about the game's lore.
The Ascaris, introduced in Vor's Prize is a restraining device used by Captain Vor in an attempt to capture a Tenno by making them subservient to him. The device is based off Corpus restraints used to keep robotic proxies obedient, and works by injecting a nano-parasite into the host, slowly taking over their physiology until the controller fully controls the victim.
In Vor's Prize, Captain Vor was seen using the Ascaris in an attempt to capture the player. His Ascaris is capable of taking over a Tenno's physiology, their transmission systems, and even shut down the Tenno's own free will, making them obedient.
As this Ascaris is based off Corpus technology, the Corpus-designed Ascaris Negator can be used to remove the restraint. Captain Vor's Ascaris possesses a fail-safe system against tampering however, making the implanted parasite detonate and kill its host unless its controller, Captain Vor, is killed.
In-game, the Ascaris' primary effect is to halve the Tenno's maximum shields, making missions more difficult due to the reduced protection. This effect only activates twice: first when the Tenno rescue Darvo from captivity, and second when the Tenno infiltrate a Grineer Galleon in order to learn of Captain Vor's whereabouts.