Saw a guy go off with this build - High Power

If this was a pro player, then he had a specific strategy in mind that he was able to implement with the help of his team. Pros build strangely sometimes based on what pro team they're playing against and I wouldn't follow any of their builds to the letter. They're great for inspiration, but unless you're playing with 3 or 4 friends, you can't expect the same outcome every time.

My Build suggestion: Shoes of the Magi, Shaman's Ring, Spear of the Magus, Gem of Isolation, Divine Ruin, Chronos' Pendant.

Not that the build is bad at all, but I feel some improvements could be made, depending on mode.

Firstly, always good to indicate which mode(s) this is for. Without a starter, I would assume this is NOT for Conquest, so my comments will be with that in mind.

The first thing that really stands out to me is Obsidian Shard in the 3rd position. At this position, enemies building protections might have some decent magical protection, but it's unlikely to be much over 100 at this point. Still, the pen against those gods should be pretty good. But your main target shouldn't be tanky gods, but other squishy gods.

Against those gods, you're looking at maybe 40 magical protections for squishies going full damage. 15% of 40 prots is 6. 6 pen is just so weak.

So a proposed adjustment would be to delay Obsidian Shard to late-game. Get it at least 4th, but maybe 5th. To make up for the lack of the somewhat early pen, change Shoes of Focus to Shoes of the Magi. Get Pythagorem's Piece 3rd after Gem of Isolation, but make sure when you get this that you're getting the full use out of it...meaning, don't go lone wolf for kills, but play a good team game with teammates close, so they can also benefit from the lifesteal/power aura. Otherwise, change out Pythag's for Spear of Desolation, unless you really need the lifesteal (in which case, get Bancroft's Talon).

To make up for the early lack of CDR, pick up the mana buff, and consider getting Potion of Magical Might.

Move Rod of Tahuti to the last position, and consider it situational, though I understand your theme of the build is the high power you mention. In teamfight modes, I feel you'll get more out of Chronos' Pendant and spamming your ult...so try that in the 4th position.

Adjusted item order would be: Shoes of the Magi, Gem of Isolation, Pythagorem's Piece, Chronos' Pendant, Obsidian Shard or Spear of Desolation (if few tanks), Rod of Tahuti (or something else).

Another build variant would be: Shoes of Focus, Gem of Isolation, Spear of Desolation or Divine Ruin, Chronos' Pendant, Obsidian Shard or Spear of the Magus, Rod of Tahuti (or something else).

I bet the guy you saw "go off" was facing weaker competition, had good matchups, or was just very skilled. This build you have is okay, but there are so many variants you could have that are also effective.