All of these have equal chances. If the entry count requirement isn't matched, try again until one succeeds or 10000 attempts were made.
At least 1 prior entry: A crossbow bolt (12dmg, AP, untyped, magical)
At least 2 prior entries: A huge ballista bolt (25dmg, AP, untyped, magical)
At least 3 prior entries: A "dozen" crossbow bolts (6 hits, 12dmg, AP, untyped, magical)
At least 4 prior entries: A burst of flame (1d50+ dmg, AP, fire, magical)
At least 4 prior entries: A deadly cold (1d50+ dmg, AN, cold, magical)
At least 1 prior entry: A sword swiped out (10 dmg, untyped, magical, fully respects prot)
At least 4 prior entries: Become cursed. This could still happen if you were already cursed when you went in.
At least 10 prior entires: A major magic discharge (1d200+ dmg, AN, untyped, magical)
At least 6 prior entries: Disintegration (999 dmg, AN, untyped, magical, MR negates)
At least 7 prior entries: Send to inferno (unresistable)