Things like these create inconsistencies in the fluff, I feel. They're the "ultimate last ever resort" yet if Horus *really* wanted Terra to fall, why the heck not throw a few Virus Bombs at them to warm things up a little? A cyclonic torpedo or two onto the Palace would've been an absolutely gravy way to tell Big E you're coming. Instead he does, what? Drops his troops and advances. Like any other general. Not like a Warmaster.
Same can be said for the Necrons. They don't need light or warmth so if they actually wanted to cut the heart out of an empire they could just turn off all the suns...! For a race with the ability to singularly destroy every other living thing they don't go about it very efficiently. Heck, the Silent King wants to kill all the Tyranids, but the only way he can think to do it is by killing everybody else so they don't feed the Tyranids and make them too strong for the Necrons. Ok, fair enough. How does he go about doing this?
Batters his army silly trying to kill EVERYBODY so even if he manages it he's still too weak to fight the Tyranids. Because of course, it'd be far too easy to just turn the suns off...