Slaughtering animals at the graves of the Awliya’ and supplicating through them
All that you mentioned concerning
offering sacrificial animals at the grave of the Waliy is an act of Shirk (associating others with Allah in His Divinity or worship). There is a detailed discussion about invoking them (other than Allah) and seeking their help for rain or anything else among similar acts of major Shirk. However, if a person asks Allah (Exalted be He) through their honor and dignity, which some people call Tawassul (supplicating to Allah through a means), then it is an act of Bid`ah (innovation in religion) which is not permissible and is considered a means that leads to Shirk. However, it is permissible for a believer to ask Allah through His Most Beautiful Names and Attributes, through Tawhid (Oneness of Allah) or through the person's good deeds. It is the duty of the ruler to ask anyone who practices major Shirk to repent, otherwise they should be killed, for the Prophet (peace be upon him) said:
''If somebody (a Muslim) discards his religion, kill him."
Related by Imam
Al-Bukhari in his Sahih (Authentic book of Hadith).
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.