Eilers, 30, died of a single gunshot wound from James Robert Malec, 48, who has been charged murder in the second degree, according to the Canyon County Sheriff's Office.
The incident happened around 10:30 p.m. at the home of Eiler's mother, the significant other of Malec. Malec was booked at 2:06 a.m. in the Canyon County Jail.
The Sheriff's Office is calling the incident a "domestic disturbance."
"We had multiple people that were at the house that we talked to this evening and on all accounts a disturbance that escalated to an unfortunate end." Lieutenant Marv Dashiell was quoted on Idaho's news outlet KTVB.
Eilers (13-7-1) is a longtime member of the Miletich Fighting Systems and in his most recent fight, challenged for the first-ever EliteXC heavyweight title in a losing effort against Antonio Silva.
Eilers, who competed for the UFC from August 2004 to February 2006, also challenged for the UFC heavyweight belt at UFC 53 in June 2005 against then-champion Andrei Arlovski.
Last week another UFC veteran, Justin Levens, was found dead from a gunshot wound to the head.