1. Go Youmuu's Ghostblade as a general Assassin item, as it is a very important item for any Assassins.

2. Eclipse is probably the best Mythic for Zed. You can one-shot and fight extended fights. I recommend going this Mythic every game with either Electrocute, Conqueror, or First Strike.

3. I don't really know if Duskblade of Draktharr is too good on Zed anymore, since you can't build it with Youmuu's Ghostblade. It shouldn't be too bad, and you can probably make it work.

1. You'll buy Serpent's Fang when the enemy team has a lot of shields.

2. Buy Umbral Glaive when you are planning to gank a lot to clear wards.

3. Guardian Angel against enemy teams with a lot of AD or Kill Pressure.

4. Buy Maw of Malmortius against Heavy AP.

5. You can also go Death's Dance if the enemy is heavy AD. If need you can go both Death's Dance and Guardian Angel.

I find First Strike better than Electrocute. The passive gives gold, and has a faster cooldown. Since a lot of the time games won't end early, First Strike will eventually outscales and outdamages Electrocute.

Run Conqueror when against a lot of melee or tanky champions. For this rune page, you usually rely on your Shadows and E to be your main source of damage. Always go Presence of Mind since you're an energy champion. Go Legend: Tenacity against CC or Legend: Bloodline against melee and tanky champions. Since you're an Assassin, you'll always go Coup de Grace, unless you are going against all Tanky champions, then you'll run Cut Down.

Go for the Electrocute rune page when against all squishy champions. You are a lot stronger than First Strike early game. So you will generally want to snowball early game.

Secondary Runes:
Your secondary rune page will be Sorcery. You will always take Transcendence because of how valuable Ability Haste is. Next is either Scorch as default. Nullifying Orb against heavy AP or Gathering Storm for late game scaling.

Q&A:
So what about Precision and Domination for secondary? These rune page is still good as a secondary but again. Sorcery just outshines them both. You can go Presence of Mind and Coup de Grace with Electrocute if you have trouble with hitting your abilities. I would recommend to never go Domination with Conqueror because you will need the Ability Haste.