If you breed it with a Ditto or another Weavile, (of the opposite gender) it should start to learn moves.
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First, do not evolve your sneasel, second, find a heart scale, third go to the move relearner's house. Give him the heart scale and select your sneasel to relearn night slash
I think night slash because more Pokemon can learn slash but few can learn night slash if it weren't a TM
Sceptile, to my knowledge, is incapable of learning Night Slash.
It is possible for Weavile to learn the following moves at the corresponding levels: Level 8 - Quick Attack Level 10 - Screech Level 14 - Faint Attack Level 21 - Fury Swipes Level 24 - Nasty Plot Level 28 - Icy Wind Level 35 - Night Slash Level 38 - Fling Level 42 - Metal Claw Level 49 - Dark Pulse Weavile can also learn a number of moves available through using a TM or HM.
Absol learns the move Night Slash at level 41. Absol also learns the Psychic type version of Night Slash called Psycho Cut at level 47.