To Clear Things Up
Eye of Skadi: This item is a potent slow. And even though it sees less play than expected, I believe this Item could help you in games were your team's lock down is minimal to none. This item provides you with the survivability and slow to chase opponents, added on to the slow from your dagger. This Item should be purchased to replace Sange and Yasha, as its effect is very similar to it.
Skill Build: Maxing dagger first is NOT always a must. The skill build is to help with the one i feel to be most flexible and effective in most cases. The dagger is very useful to last hit with in the early game, and this is why you get Ring of Aquila as a core item in the 'Fighting' section. It is not found in the 'Farming' section because if you get space to farm, there is no need to spam your daggers for last hits in the first place. It'll just delay you Battlefury.
Maxing your blink effect next is simply for the flexibility it provides. It is a good escape tool (4 seconds CD at max lvl), and a potent offensive tool to harness early on.
So, why not max your passive first? Most of the time, you enemies won't have enough physical burst for your passive to become useful. Someone like Clinkz might make you skill it, and that's completely fine. However, the spell isn't as useful early on as you might perceive it to be, having less uses and serving really no purpose at all. Maxing it later will make sure you skill your more flexible and important skills first.