This build is a damage focused build with as much Penetration as possible for you to deal damage, as the enemy comp may be building protections to counter you as well as any other physical gods on your team.
With the combination of Jotunn's Wrath, Brawler's Beat Stick and The Crusher, you have more than enough penetration to deal as much damage as possible with your abilities due to Ratatoskr's high burst.
However if you want higher burst damage in exchange for a bit less CDR, pick up Transcendence instead of Jotunn's Wrath as all of Ratatoskr's abilities have some pretty high scalings. 70%, 100%, 90% and 70% respectively!
If need be, replace Brawler's Beat Stick with Winged Blade if the enemy team have slows. Keep in mind Heavenly Wings will have you immune to the effects of slows, but they will still linger. So if you are slowed during usage of Heavenly Wings and it's still there at the end, you WILL be slowed so be careful!
You have some protections and health with Magi's Cloak but not a lot of it, so you are still rather squishy and you should not be diving in recklessly unless the opportunity for the perfect gank is available and you have enough time to escape.However with Magi's Cloak you can avoid hard crow control + roots due to the bubble without having you use your beads.
This build is good if the enemy team has high burst combos coming from magical characters like He Bo, Ao Kuang or Scylla for example.
Due to having flat reduction with Flurry, you can opt into this hybrid build with Titan's Bane as your only penetration item, deal a LOT of damage and still be tanky enough to not die immediately. You roughly have 200 of physical protection and just over 170 magical protection.
The goal of this build overall is to essentially harass the enemy squishies; the Mid and the ADC, the Jungler too if they don't adapt a more tankier build and build full carry plus Magi's like the other build shown above.
Which to go for...
So the real question is, which of these two builds do you go for depending on the situation? (Applies to non-conquest modes specifically) If your team lacks a tank, then the second build is just for you, providing plenty of both protections as well as a 90 second crowd control bubble due to Magi's Cloak
if the enemy comp has a lot of crowd control, which can hurt Ratatoskr's
mobility if he's locked down.
However, if you want to be a main source of damage for your team and take out squishies pretty quickly then go for the first build. The sheer amount of flat pen you have (40 in total if you choose to not take Winged Blade
!), is massive especially in the early-mid game then with the percentage pen from Titan's Bane
for the transition into late game.