What was Horus plan if the Eisenstein did not escape.
Something has recently occured to me after reading The First Heretic and a passage in Galaxy in Flames. If the Eisenstein never escaped to carry word of Horus treachery to the Emperor was did Horus plan to do? Because in Galaxy In Flames after he learns of the Esiensteins escape he emphasises how critical the Istvaan V phase is, which of course does prove to be the case as it near destroys three loyalist Legions. The Word Bearers aswell also know that around the time Istvaan III is happening they will soon get a call for them to support other loyalists at Istvaan V where the betrayal will happen.
But if the Eisenstein never escaped Istvaan then how would the Imperium know that Horus had betrayed the Emperor and send a host of Legions to confront him on Istvaan V. If he always planned Istvaan V to play out how it did then what was the problem of the Esienstein carrying the warning away. The loyalists would have known something had happened at Istvaan III because of the phycic death scream that apparently was so bad it blocked out the astronomican, but without the Eisensteins message they could have taken alot more time to gather forces to be sent to aprehend him, the other Legions could be given more time to regroup and prepare themselves and be better placed if an attack happened. And the Iron Hands wouldn't have been able to arrive in full force instead of rushing so much.
I dunno, it just seems that if anything the Eisensteins warning helped the Istvaan V phase go even better.