As you can see this item build is more general than specific I would like to point out some different combinations.

a good item before your mercury treads may be wriggles lantern, this depends on wether you have a jungler or not, i find that this allows you to get red buff, ward and take down dragon, and help team mates get blue (always help your team out). Late game I trade this for blood thirster. If you get this item don't get zekes.

I find that frozen mallet is a good item to buy, it allows you to chase enemys effectivly and makes sure that they are stuck in your ult. The hp is also nice for atmas, when combined with warmogs. This also allows you to survive when in ult and tank for your team.

Now, in late game, you should be able to identify the major damage threats. If its AD related get thorn mail, if not get banshee's.

ganking: when ganking always do it from a brush, if possible. Then use decoy and go in, the extra second where you are in stealth can help you get in range for nimbus. Your ult is also a good tool to use because of the knock up, always try and stay infront of them, so that if they flash you are still in range.

team fights: if you have a tank, wait for him to initiate and then use your ult, try and go for the squishys in the back and hit everyone for the knock up. My item build also allows you to initiate fights, just make sure your team can commit to a fight too.

saving allies: if you notice an ally is getting chased while in your range nimbus to the enemy and hit em with a few basic attacks with frozen mallet. If you feel that this isn't enough you can use your ult too, never hesitate when doing this unless the whole team is after him and you are positive hes dead; lots of times I have saved an ally with this, or even kill the follower too.

pros:
very tanky
can use ult in a team without much worry
slow from mallet and nimbus is very effective at chasing
zekes helps your team, adds some ad from atmas(5 ad) and gives you cd which is useful
all around helpful character that makes those game changing aces
trinity from other builds isnt very useful with your ult

cons:
not as much damage (thats about it)

This is my first build and it is a rather general one, give me plenty of feedback on my guide and stuff that I should add. A spelling and grammer lesson is okay too. Eek Eek Ook