The only TV Series based on The Exorcist characters was quickly squashed by Blatty, Friedkin and Mark Kermode in May this year. It was announced in the trades as 'in development' but they can't distribute anything involving Blatty's characters without his blessing, as I understand it. Here is the article with more details.
Back in 2009 Blatty confirmed he DOES HAVE a miniseries teleplay written, which Friedkin had agreed to direct, but there's been no word of production going ahead.
I'm not aware of any other TV series' based on The Exorcist, but there may be others that involve possession in development.