Witch is dark troop and is available for unlocking at Town Hall 9. Witch is one of the most powerful troops if used correctly. Witch raises skeletons that are not very powerful but are useful.
Description as mentioned in Clash Of Clans: The Witch never fights alone, constantly raising dead warrior from past battles to lead her attacks. Upgraded Witches raise more skeletons at a time.
Witches are usually known for GoWiWi attack strategies. GoWiWi was one of the most powerful strategy in past. But nowadays GoWiWi attack strategy is not used.
Currently Witches are being used with Bowlers. Many legends use this strategy to 3 star maxed bases. Personally I don't use witches but trust me this is the strogest meta nowadays.