Making Du`a' to others instead of Allah such as Awliya' (pious people) to remove harm, cure the sick, or secure a road is Shirk Akbar (major form of associating others in worship with Allah) that takes its doer away from the realm of Islam. Allah (Exalted be He) says:
And the mosques are for Allâh (Alone): so invoke not anyone along with Allâh.
He (Exalted be He) says:
"And invoke not besides Allâh, any such that will neither profit you nor harm you, but if (in case) you did so, you shall certainly be one of the Zâlimûn (polytheists and wrong-doers)."
Claiming the knowledge of the unseen is an act of Kufr (disbelief). Allah (Exalted be He) says:
Say: “None in the heavens and the earth knows the Ghaib (Unseen) except Allâh”
Slaughtering for a righteous person to seek their blessing is not permissible. It is a wrongdoing that incurs curse; it is authentically reported that the Prophet (peace be upon him) said:
Allah curses the one who sacrifices to anyone other than Allah…
Allah (Exalted be He) also says:
Say (O Muhammad صلى الله عليه وسلم): "Verily, my Salât (prayer), my sacrifice, my living, and my dying are for Allâh, the Lord of the ‘Alamîn (mankind, jinn and all that exists).
"He has no partner. And of this I have been commanded, and I am the first of the Muslims."
(Part No. 1; Page No. 142) In case that slaughtering is just out of good hospitality, feeding Muslim brothers, and doing well, there is no harm in doing so.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.