Priscillian was the first Christian (that we know of) to be ex-communicated (disfellowshipped-hehe) permanently. What for? - The old excuse of heresy (apostacy).
What did he do wrong? Priscillian did have rather extreme views - but they were views also held by others in the early Christian church and later.
He was an ascetic mystic and regarded the Christian life as continual intercourse with God.
His favourite idea was Saint Paul`s "Know ye not that ye are the temple of God?" (I Corinthians 6:19)
He argued that to make himself a fit habitation for the divine a man must, besides holding the Catholic faith and doing works of love, renounce marriage and earthly honour, and practise a hard asceticism.
His ascetic beliefs were not unusual - in roughly that period we can find Christians who lived their lives out on top of a pole. Others went to live in the desert and spent the day in prayer. Plenty of other Christians also thought they should be celibate, even if they were married.
His concept of Christian life seems to have reached crisis point when a couple of other Bishops - Istantius and Salvian adopted his ascetic views.
Two of his enemies, Bishops Hyginus and Hydatius, persauded Pope Damasus I to hold a synod (a sort of Judicial committee) to inquire into Priscillian's apostacy, the Synod was held in Zaragoza in 380 in the absence of Priscillian and his supporters, and they (predictably) were ex-communicated. Priscillian's friends then appointed him Bishop of Avila. Their enemies appealed to the Emperor (Gratianus-a Christian) who upheld the ex-communication decision. A delegation then went to Rome to try to persuade Pope Damasus to change his mind. They failed. The Emperor Gratianus was murdered at this point and in the legal and political shenanigans that followed a decison was obtained to execute Priscillian and six of his supporters. The killing (beheading) was carried out at Triers in 385.
They were the first Christians to be executed by the Christian church. (I repeat, that we know of) And that occurred less than one hundred years after the end of persecution of the church by the Roman state.
See any similarities to the way our former loving brothers and sisters operate?