There are two powers in this world; God who is all powerful and Satan who's power is limited by God.
When Satan/Lucifer fell from Heaven he took a third of its angels with him. These fallen angels are sometimes called demons.
^Revelation 12:4ESV
His tail swept down a third of the stars of heaven and cast them to the earth. And the dragon stood before the woman who was about to give birth, so that when she bore her child he might devour it.
Revelation 12:9ESV
And the great dragon was thrown down, that ancient serpent, who is called the devil and Satan, the deceiver of the whole world—he was thrown down to the earth, and his angels were thrown down with him.^
There is no communication between the dead and the living. Nor can the dead or the living "cross over" to the other side.
^Jesus said, “There was a certain rich man who was splendidly clothed in purple and fine linen and who lived each day in luxury. At his gate lay a poor man named Lazarus who was covered with sores. As Lazarus lay there longing for scraps from the rich man’s table, the dogs would come and lick his open sores. “Finally, the poor man died and was carried by the angels to sit beside Abraham at the heavenly banquet. The rich man also died and was buried, and he went to the place of the dead. There, in torment, he saw Abraham in the far distance with Lazarus at his side. “The rich man shouted, ‘Father Abraham, have some pity! Send Lazarus over here to dip the tip of his finger in water and cool my tongue. I am in anguish in these flames.’ “But Abraham said to him, ‘Son, remember that during your lifetime you had everything you wanted, and Lazarus had nothing. So now he is here being comforted, and you are in anguish. And besides, there is a great chasm separating us. No one can cross over to you from here, and no one can cross over to us from there.’ “Then the rich man said, ‘Please, Father Abraham, at least send him to my father’s home. For I have five brothers, and I want him to warn them so they don’t end up in this place of torment.’ “But Abraham said, ‘Moses and the prophets have warned them. Your brothers can read what they wrote.’ “The rich man replied, ‘No, Father Abraham! But if someone is sent to them from the dead, then they will repent of their sins and turn to God.’ “But Abraham said, ‘If they won’t listen to Moses and the prophets, they won’t be persuaded even if someone rises from the dead.’”
Luke 16:19-31^
The devil and his demons are tactful and seductive. If a spirit isn't from God it is from the devil.
Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world. By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already. Little children, you are from God and have overcome them, for he who is in you is greater than he who is in the world. They are from the world; therefore they speak from the world, and the world listens to them. ... 1 John 4:1-6
So essentially anything heard from a medium is made up or comes from the devil. I wouldn't welcome that demonic influence into my life.
If you want to find out more about angels I can post you a brilliant book by Billy Graham if you PM me your address. It goes into a lot of derail. I would also like to encourage you to look towards the creator and not the created, and the ultimate source of the angels power.