Ktarian game
The Ktarian game was a single-player headset device worn over the ears and using a visual interface. The game displayed a grid pattern on which the player had to direct hovering disks into cone shaped objects over the course of multiple levels. 

The first level consisted of only one disk and one cone, the second level had two cones and disks, and the third level had three of each. The game had at least 47 levels.

In 2368, Etana introduced it to William T. Riker for the purpose of gaining control of Starfleet, while Riker was vacationing on Risa. Riker then unwittingly helped spread it aboard the  while en route to the Phoenix Cluster.

Wesley Crusher and Robin Lefler analyzed the game using a neurological behavior program in the engineering laboratory aboard the Enterprise and found that it was actually a mind-control device that rewarded a player's successful attempt by stimulating the pleasure center of the brain, creating a psychotropic addiction affecting higher reasoning and rendering that player completely submissive. It also activated the reticular formation and increased synaptic activity in the frontal lobe and prefrontal cortex. If a victim actively resisted playing the game, as was the case when the device was forced onto the head of Wesley Crusher, the game would "play itself", and the cones would extend outwards to envelop the disks all on their own, triggering the same pleasure responses and getting the victim addicted to the program just as if he or she had been playing along. 

Etana's goal was apparently to covertly take control of the Federation by distributing the game throughout various ships, starting with the Enterprise and spreading out from there to other ships; to this end, she arranged for Riker to first hand the game over to Counselor Deanna Troi and Doctor Beverly Crusher, who could then deactivate Data – the only crewmember completely immune to the game's influence – and continue the game's distribution unimpeded.

Fortunately, Wesley Crusher was able to deduce what had happened during a brief return visit to the ship, and was able to repair Data and distract the attention of the brainwashed crew long enough to give him time to work out a means of negating the game's influence. Data then used an optical burst from a modified palm beacon to counter the effect of the device on the bridge crew, subsequently programming the ship's internal lighting to broadcast the same frequency throughout the ship. ()

Kerner Hauze had a headset from the game in his collection by the time of his death in 2381. He displayed it next to a bust of Jean-Luc Picard, which was originally created by one of the children on the Enterprise for Captain Picard Day. ()

A  crew member was wearing a Ktarian game headset at the bar during R&R time in 2381. ()

In 2383, Dal R'El played the game aboard the . The hologram of  referred to it as a "silly little cone-and-disc game." ()

Background information 

This game was actually inspired by a real game. Recalled Fred Bronson, "It was based on the fact that I had a Tetris game on my computer at home. Whenever I wanted to [...] distract myself I would play Tetris and it was very addictive." (Starburst Special #29, p. 58)

The headpieces were created by property master Alan Sims, using telephone headsets. (Star Trek: The Next Generation 365, p. 226)

Riker actor Jonathan Frakes was disappointed with the computer graphics used to depict the game. He stated, "They told me it was going to be this incredible graphic, and all it was... was a tuba on a checkerboard." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 231)
Recreation