Any apparition that appears from beyond the grave is demonic in origin no matter who it claims to be.
2 Corinthians 11:14-15 And no wonder, for even Satan disguises himself as an angel of light. So it is no surprise if his servants, also, disguise themselves as servants of righteousness. Their end will correspond to their deeds.
In 1 Samuel, Saul seeks a medium to conjure up the spirit of Samuel. From the text it is clear that it was God, not the medium that made Samuel appear. Samuel condemns Saul for what he did. We also have this from 1 Chronicles: 13 So Saul died for his breach of faith. He broke faith with the Lord in that he did not keep the command of the Lord, and also consulted a medium, seeking guidance. 14 He did not seek guidance from the Lord. Therefore the Lord put him to death and turned the kingdom over to David the son of Jesse.
It clearly states seeking counsel from anyone other than the LORD is forbidden. Even from Samuel a prophet of the LORD.