Rugal is trying to overload the Siphon with power from The Warp and possibly guiding a Warp Entity(a weaker one) to attack/possess the Siphon. I left the results up to you as the wide variety of them means that Rugal can't pick and choose what exactly is going to happen. This can mean the Siphon can go insane, go comatose, spontaneously mutate or any number of nasty things. Up to and including daemonic possession or a massive explosion causing the Siphon to turn into a fine pink mist.

Or nothing could happen if the Siphon is able to snap off the connection or has a strong enough will.

At the very least though, there's going to be a strong magical/psionic shockwave centered on the Siphon since it's like turning a firehose on.