The Build

The idea of this build is to be kind of oppressive and annoying, Grux's passive gives great omnivamp, so helps with sustain in fights

The Items

Boneseeker
Mainly for the attack speed and extra damage to targets, the health stacks nicely with bonesaw when you get it online as well

Draconium and Void Helm
Kind of essential with the build for the sustain needed to stay alive, both offering increased healing stacks really nicely with Grux passive.
Swap the build order of Draconium and Void helm if you need the specific defence.

Bonesaw
The passive on this item helps Grux in 2 ways
1- Increased damage that scales off of your health (you have a lot of it)
2- A possible extra 30% attack speed (which is lacking in the build, outside of using your ultimate ability)

Mutilator
Helps insanely well once you get it, the health shred on basic attacks is key for this build as there isn't much damage coming from elsewhere. Plus you are using abilities a lot on enemies, so stealing 5% of their health over 20 seconds feels super nice in big fights

Flex Items

Sky Splitter
You can replace Boneseeker with Sky Splitter if you want a bit more damage, movement speed will go down slightly. (You will also lose some damage from bonesaw as you won't have as much health for it to scale from

Warden's Faith
Self-explanatory, take this if you're struggling with enemies doing crit damage to you

Tainted Guard / Bastion
If you need anti-heal and your support doesn't already have any of these, I recommend getting either of them (depending on damage type)

Ability Upgrade Order

I recommend maxxing out your Double Pain as soon as you can, this is where most of your damage will come from, it also applies on hit effects twice, which is nice to get Boneseeker, Draconium and Bonesaw stacked quicker

From there, upgrade your ultimate whenever it lets you, and the other 2 abilities you can take how you please