Bring your horse to a Wayshrine and make sure you have more fame than infamy.

Then activate the Wayshrine and be assured no pillar is behind you and that your horse is standing directly behind you.

Then when the magic is shot out of the altar make it hit your horse instead of you.

You have to jump out of the way as soon as it says (rejoice, your afflictions are banished).

Jump and hope that it hits.

If you partake in this your horse will be forever stronger and faster (for example my horse ran from the base of Diverock to the Imperial City in 5 minutes).

[unconfirmed]