Why? This stacks, and is useful because Thor is mana hungry. This will also fit with the build.
Why? This is your basic starter for the match. IT gives your power for early game. After transcendence is even more powerful.
Why? This will combine power with transcendence with the mana, early game penetration. And this is useful because your ultimate will take 90 seconds cooldown. This will reduce cooldown.
Why? This gives extra mana and combines with transcendence, gives you full cooldown. And since his 3 is powerful already, maybe grab a protection. You scrub.
Why? The passive of this item combined with your own passive. Do the maths: 3 Hits = Full Item Stacked 30 Physical Protection down, with your own passive. 30 Physical Damage. That means you add 60 extra damage to your basic attacks, and this penetrates through armor which means your ultimate does 200 extra damage. Your 1 does 150 extra damage with both hits, and your 3 does 10 more damage per tick.
Why? This gives your a chance against tanks, and with your stone cutting sword maybe even kill a tank. This also comes with physical power.
Hey this is my first guide, of course it wouldn't be too good. But I hope the build worked.