Hey Legends,
I wanted to share some notes on items when playing the Unkillable Demon that is Serath.

So first of all, this is an item guide and as such, there's no real "Set build". Seraths itemization is pretty fluid and it may be different every game; build what you need, and try out some variations to see what works for you.
I've listed alternative offensive/defensive items that I have had success with, these items cover best-in-slot choices to counter your opponents.

Now to the 'Core build'

Sky Splitter -
Literally my favorite item on Serath.
All the stats you want, it provides so much damage and survivability.
Won the teamfight but you need more gold for your next purchase? Farm a camp or 2, you'll be back to full hp in no time and ready to grab the free pile of gold from Duo (AKA. The enemy Carry).
The passive will also help those first hits REALLY hurt dealing 3% current health on hit. 10/10 item

Bonesaw -
So originally I wasn’t completely sold on this, but the spooling attack speed and giant chunk of HP are just too good. Stacking health also allows for those big chunks every 3rd attack, you’ll now deal a 2% of your health on hit, on top of a 25% slow. Prrrrrrettttttty damn useful.

Basilisk -
With the 2 items above, we’ve completed the holy trinity. Every attack you shred armor, once the stacks are maxed you’ll burn through targets with 3.5% missing health on hit.

Draconum -
Now I know this item needs no introduction. Fantastic stats, boosts Healing to mitigate any Tainted items that have been built, Health back on takedown.
What's not to love?

Final 2 Items -

These are the slots 'Reserved' for items to round out the build, again your entire build should be flexible, build what you feel you need.
The two items I find myself building often are:

Absolution -
Damage Mitigation / Tenacity are key for CC heavy teams.

Salvation -
Amazing Anti-burst item with Tenacity and HP, also Omnivamp and a Shield when below 40%. [Chefs kiss]

Both are fantastic and work with a multitude of possible item combinations.
If you aren't having trouble surviving teamfights and would like extra damage for that true 1-shot experience, pick from the list of Damage Replacements.
IE. Vanquisher watch your enemies pop in teamfights - Enough said.

Alternative items

Damage

Vanquisher -
Now Vanq is controversial, but I believe it's GOATed.
You'll catch so many people.
It makes tower-diving easier (Lets be serious, this is the reason you play Serath), synergizes amazingly with your burn, allowing you to pickup kills that may have flashed / dashed away.
However, this is the first item to change out if you NEED something to deal with the opponents comp.

Nightfall -
I don't know why some people are still sleeping on Nightfall. Another amazing option that provides so much Bang-for-Buck.
Damage, Omnivamp, Pen, Heal on ability and A SHIELD ON TAKEDOWN with no CD!? 100% OP Item.

Demolisher -
This is the ultimate 'the bigger they are the harder they fall' item. Damage scales based on the HP difference between you and your opponent, so if you find yourself against 2+ thick bois on the enemy team, this may be worth the pickup

Demon Edge -
Do they have shields? Boy do I have the item for you!
For real though, you if you're having problems with the muriel shielding the hell out of your target, grab this.
It gives a move speed stim and extra damage to help you get past those pesky shields.

Note on Alternative Items -

I'm not a massive fan of 'When attacking deal X damage - CD'or 'Build 100 stacks for effect' items (IE. Spectra/Stormbreaker), I know they work, but I prefer the consistency of the flat damage items. More testing to be done in the future.

I'll update this build with item changes and different items I find useful.

Good luck out there Legends.