Horus Heresy Collected Visions pg. 368

Rogal Dorn knew something was wrong. He had materialised on one of the
lower decks of the great battle-barge that was the flagship of Horus. The
Emperor was not with him. Nor was the Angel Sanguinius. The Custodian
Guard, the Emperor's sworn bodyguard, surrounded him and instantly
adopted a defensive posture.
'What has happened?'one of them asked Dorn.'Where is the Emperor?'
'It seems we have been tricked. Horus has worked some fell magic to separate
us,'the primarch replied.
''We must locate the Emperor at all costs, and do
it quickly,'he added.