What is meant by Sufism or Tasawwuf, and what is its ruling according to Islamic law, along with the evidence and explanation?

The title of Sufism was given to some ascetics and devout worshippers in early Islam. Then Sufism became mixed with innovations (bid'ah), extremism, and forbidden philosophical doctrines such as Wahdat al-Wujud (unity of existence) and Hulul (divine indwelling). Contemporary Sufism includes deviations such as exaggeration in venerating شیخs, worshipping at shrines, reviving mawlids (birth anniversaries) and innovated litanies (awrad). The doctrine of Hulul and Ittihad (union with God) still exists. True Sufism is asceticism and worship in accordance with what Allah has legislated.