Leave it to Providence
It must have been luck.
The first few million Angyls destroyed by seemingly natural phenomenon were dismissed as an accident by the Legions. So much power manifested on a single world it was understandable that it buckled beneath their magnificence. To be expected even, a worthy tribute to their majesty.
Then billions started to die as the natural phenomenon grew in scale and ferocity, as the planet itself rejected their coming.
Storms and tornadoes darkened the heavens, as the world roared before their coming, tsunamis of silver shards danced through the skies to flense angelic skin and wings, burning their essences to the core as the pure metal cut deep.
But although the Angyls searched and hunted they could find no cause of these calamities, no source for their tormentor or means to relieve themselves upon it. For what was their heckler but the wind and the ground? Even the daemons in the materium could not strike the earth and expect it to bleed, nor could they command the wind to cease its blowing no matter their power, for all that the imperious daemons did so, expecting to be obeyed.
All they could do was endure the convulsions of the earth as ash and dust was thrown up in almighty quantaties to block their path, volcanoes long invisible or dormant speaking forth their molten song, as Magma Wyrms and tidal waves rose in anger and outrage.
But of course there was a source to all this madness, beyond the sight of god and men. Deep within the world's belly, lay the one who controlled it all. Providence the World had named it, the greatest builder of them all, from whose mind entire worlds were spun, picked out in glass and jewels in the heavens above.
Long it laboured and had tinkered, readying itself for this gilded day. Preparing the region for the incursion that would soon hold sway above it, the cataclysm that would break it down to its foundation.
Through secret paths and mystic lore it built its troves of hidden traps, super volcanoes and mega hurricanes, tidal waves and world shaking earthquakes and more, all the fury of the natural world that it could possibly bring to bare, in a form that the angyls would never be able to resist completely.
It turned the advantage of a few years anticipation and created a series of ambushes so devious and duplicitous that the Angyl's assault was slowed down by hours, their strength cut down as they approached the defended city, minute wards carved invisibly into the ground activating around it, shielding it for the final fight.
It would have horrendous effects on the laboratories, but Providence had used its time to prepare for that as well. It could rebuild them, better and stronger than they had been before.
And when the Archangyl reached the spine, it had a special surprise waiting for it.
The Sin of Man nor really did man itself, know of the true power of the World.
Perhaps a taste of it escaping through a new vent would give them a small lesson in what true power really was?
Providence is quite a fickle mistress at times, its easy to state that everything is predetermined, but sometimes it really is and it can drive you insane.
@Durin