Little Bit on Why

These items work for most offtank or tanky bruisers. The core items are always great, Overlord is a must have for split pushing and really maximizing any high hp hero. Fire blossom is a pick I personally really enjoy, but its something that should only be chosen if you are winning lane, its passive pressure will continue your snowball. In general building tank on greystone while youre ahead will be more effective than damage since his kit allows him not need to back to base often, so turning yourself into someone who can apply constant lane pressure and having enough tank to escape when you get inevitably ganked is key (plus the more health you have the better your ult is).

Skill Order

Make way (spin move) is the one to max first. Then sacred oath. Then assault the gates. Take your ultimate when you can.
Try not to engage with Assault unless you know you wont be ganked (vision on mid and jungle), its also useful to save so you can jump on your target when they try to run. You can just pop make way and sacred oath at the same time when you engage, try to time sacred oath right after finishing a normal attack. Youll want to activateSaphir's mantle before you ult to take advantage of the extra health (mainly for late game team fights). In early game try to pop it in the middle of minion waves or jungle camps with a 5 minions to max out the gains.

Some general tips:

Your ult makes you invulnerable, use it to counter other ults or burst options. You also want to be the one to engage team fights for your team and using it when they begin to focus you can allow your team time to set up combos or just get free hits in. Assault gets a 35% reduction if you hit a hero so dont miss it, and be careful about using Make Way to kill creeps when you have a minion wave, you can lose out on last hits, you should also be careful about hitting the enemy when they have a full creep wave so you dont get the aggro and take useless dmg.