Muramune is physical now by the way

Pretty nice guide since you gave us different options against different opponents. Could use some added information, though, and some editing to please the eyes. :)

also, does muramana still activate when you use your ultimate?

I like the build but I'd take iceborn gauntlets over frozen heart. It gives you 5% less CDR, but with your masteries plus passive on Q you're still at 35%, and blue buff or blue potion puts you over the cap. You lose 30 armor and the attack speed reduction passive, but gain 100 mana, 40 AP, and AE spellblade, dealing 125% damage to surrounding enemies on a basic attack after using an ability, plus slowing everyone in the area for a few seconds. With Muramana, you're over 200 atk dmg, so that's an extra 250ish AE dmg every 2 seconds in a teamfight plus the slow. If nobody else on the team can do frozen heart and you're up against a strong AD team get it, but otherwise iceborn gauntlets is better.

Also, I like scaling MR runes way better than flat MR. At lvl 6, the scaling ones overtake the flat ones. Llandry's torment is also a really good item, but it's hard to give up any of the stuff already in the build. If you're really facerolling and they don't have much magic damage or CC you can replace banshee's veil with it, or if the game is looking like it will end before you can get your seraph's staff you can get it instead of that.

The comparison of muramana/seraphs could use a little more work. At 18, with the iceborn gauntlets, you have 5020 total mana. At full mana with the full build, you get about 300 bonus damage on each spell or basic attack. That it works on basic attacks is important to emphasize too, especially withe the iceborn passive thrown in.