There have been no deaths in the UFC history. As for MMA in general, there were 7 deaths in sanctioned fights and 9 in unsanctioned fights.

Are there any deaths in UFC?

As of April 2019, there have been seven recorded deaths resulting from sanctioned Mixed Martial Arts contests and nine from unregulated bouts, none however in the largest MMA promotion Ultimate Fighting Championship.

Is UFC life threatening?

Yes, fighting in MMA matches is dangerous just as in any other combat sport out there like boxing or Muay Thai. It differs a lot from training since the goal is to hurt the opponent as much and as fast as possible. Pro MMA fighters throw each punch or kick with violent intention to knock the opponent out cold.