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Does damage equal to the target's max HP. Will only work when user has a higher Speed than the target. Using moves that alter Speed can allow the Pokémon to use Horn Drill successfully.
None, as OHKO moves are banned under the OHKO Clause. Some Ubers matches allow the use of OHKO moves, Mew being the main user of them.
Fissure is a Ground-type equivalent.
Guillotine is an equivalent.