Exact skill check odds and Monte Carlo combat for both games, with the rules ported from the reconstructed source and the game's own random number generator. Everything runs in your browser; a save's PLAYER.DAT can be loaded and never leaves it.
Ultima Underworld: The Stygian Abyss
Nine levels, 657 monsters, UW1's casting, experience and enchantment rules.
Ultima Underworld II: Labyrinth of Worlds
Eight worlds, 746 monsters, UW2's casting, experience, special weapons and timing.