The painting that Maris is helping to re-enact, "Martyrdom of St. Ursula by Attila the Hun" that Niles mentions is an actual mural by Johan Schmitt (1825-1898), painted from 1868-72. It is one of several murals commissioned by the Ursuline Convent in Louisville, Kentucky. The description Niles (David Hyde Pierce) gives of the painting is indeed accurate.
This is the last episode in which Niles's Apartment is shown.
The title is based upon the quote from William Shakespeare's "Hamlet" Act I, Scene V (lines 27-28) where the Ghost speaks: "Murder most foul, as in the best it is;/But this most foul, strange, and unnatural." It was also used as the movie title Murder Most Foul (1964), based on a novel by Agatha Christie.
Deanne Berry, the fitness instructor featured in the iconic "Call on Me" music video, also makes a cameo appearance in "Murder Most Maris" as one of the aerobics class participants.
As Niles rises from his cafe seat, and into the apron Frazier is holding for him, the edge of flesh colored bikini briefs can be seen on his hip.