I've been looking at guides on Solomid. It looks like Phage is one of his cores. But from Phage, what do I build? Do I go into Trinity Force (as the first big item) or do I build it into Frozen Mallet. I've seen suggestions that he should get TF first then Frost Mallet but how viable is this exactly (sounds like quite a bit of gold if you are not dominating).
Depends on how your team is doing and whether they need an off-tank or more damage.
While Trinity is a very potent item and Darius uses almost everything it gives him very well, going for a Hexdrinker after a Phage might be better.
Your mid losing his lane? Go Maw and stomp that mid. Bot losing lane? Frozen Mallet + Atma's.
Ideally, you always want to build Darius with both damage and resistances, along with some HP. He needs to be in the thick of things, so everything helps him.
My preferred core build with him is: Mercury's Phage->Mallet (Blooodthirster after Phage if yo're utterly stomping your lane, hyper snowball) Atma's Impaler Hexdrinker->Maw Guardian Angel/Bloodthirster Trinity if you can afford it, rounds up his build (Phantom dancer as an alternative for even more AS/MS/Crit chance than Trinity, but you're losing out on extra mana and Sheen procs that way)
He's actually very flexible as far as builds go. You just counter-build. Enemy team has multiple heavy sources of AD damage? Go Frozen Heart. As for Maw, it shout always be in your build, that's simply a non-issue. It'll save you from an AP mid's burst more often than you could imagine and actually make you the biggest threat to that AP mid.