Complex sentence: The dagger is tainted, as is every item and pebble in Shadar Logoth, but before Moiraine realises this, it is too late, and the dagger has bound itself to Mat, effectively tainting him.
Split into simpler sentences: The dagger is tainted, as is every item and pebble in Shadar Logoth. Before Moiraine realises this, it is too late and the dagger has bound itself to Mat, effectively tainting him.