Professor Phobos said:
Once per session, at a random time designated by me, a knight will appear and- for completely arbitrary reasons- have his neck snapped.
Your DM fiat poses a high probability of incompatability with gamist players, with commensurate increase in inter-player conflict and danger of campaign collapse. I suggest the use of the following chart.
Table I.A. (roll d100)
01-13 knight falls off horse, snaps neck
14-24 knight is in brawl with thug, thug snaps knight's neck
25-37 knight is hit in neck by flail
38-41 knight is hit in neck by flying cannonball
42-54 knight is wrongfully sentanced to death by hanging
55-58 knight trips on own feet, falls and snaps neck
59-63 knight eats quall's feather token:tree, says command word while token is stuck in throat
64-69 knight misses ball with croquet mallet, hits self on neck
70-73 knight forgetfully lifts off own head instead of helmet
74-77 knight wanders into anti-gravity zone, falls up into tree
78-83 knight slips on ice patch, slides head-first into wall
84-86 knight has accident with handglider
87-90 summoned rhinoceros materializes over knight and falls on knight's head
91-92 knight is reincarnated as a badger, snaps own neck in shame
93-94 knight is hit on head with rubber chicken, knight lives!
95-99 knight encounters wandering prostitute, roll on table I.B.
00 roll three times on this table