Mundo is capable of a lot of things, and resurrection is one of them. To give you a basic idea, it's pretty much the same concept as Tryndamere's ult. The art of resurrection makes Mundo, who is already notorious for being extremely hard to kill, unkillable even when he dies. I'll explain my other various choices as well throughout the guide.
NOTE: This is not a jungling guide, but can be built pretty much the same way if jungling with a little change in your skill order, which I explain later.

Before I get started, I want to say that this build makes Mundo pretty much UNSTOPPABLE in 3v3s. It is very viable in 5v5s but REALLY shines in 3v3 match-ups.
With that said, lets get started.

The runes are not set in stone; I believe that runes, like summoner spells, depend on how your playstyle is. The only ones I would say are necessary would be your Greater Seal of Vitality x9 and Greater Mark of Desolation . This allows you to be tanky from the start of the game, and have early game strength to get a faster Warmogs, which is good.

I go 21-9-0 simply because that is as far as you need to go in the defense tree to benefit from it. The 21 points in offense is drastically more helpful than if you ran a 9-21-0 build. This can work with a 9-21-0 mastery build though. This depends on your playstyle. If you are less aggressive and more on the tanky playstyle, or your team lacks tanky-ness, 9-21-0 is probably better for you.

With this opener you can feel free to jungle if you like, or lane. Some people prefer jungling with mundo, as he makes a great jungler. This build is also great for solo laning top if you already have a jungler, and is fantastic in top lane 3v3s. For boots, choose Mercury Treads if they have a lot of AP or CC. However, I much prefer Ninja Tabi and here's why; Your W gives you 35% CC reduction already which is pretty much a built in Mercury Treads. The overall damage reduction percentage that Ninja Tabi provides + the armor boost scales very well into late game and is cheaper than Mercs.


I get Warmog's Armor first for 1 main reason. Your lvl 1 ultimate heals you for 40% of your max health and the Spirit Visage increases this to 46%.
If you get Warmog's Armor instead, even if you just buy Giant's Belt and Ruby Crystal you'll have about 400 more health as you'd have with Spirit Visage .Early game 400 health increases your ultimates heal much more than the 15% healing effect increase from the Spirit Visage .

Then after Warmog's I buy Spirit Visage :) It scales better with level 2 ult +

Atma's Impaler is self explanatory when you have 4.5k health.

The Guardian Angel is what makes you the Messiah. IF, and when, you are about to die, pop your ult right before you go down. While Guardian Angel is resurrecting you, your ult heals you to almost full hp and you live to fight another battle. Stay alive for around 48seconds give or take and your ult will be up yet again and you can just continue baptizing kids with your Blunt Force Trauma .

If the enemy team has at least 2 AP, or their AP carry is doing well, then Force of Nature is the item for you. The built in health regen just builds so so well with your passive and Warmogs, and this item gives the most Magic Resist in the game.

If CC is your thing and the game is going swimmingly then go ahead and buy the Frozen Mallet. There are a number of other items you can get, such as one of my favorites, Phantom Dancer . This item allows Mundo to go where he pleases and parade around like a drunken Yunt leaving nothing but a trail of blood tears and sodomy in his wake.
Rainduin's Omen is also a valid choice.
Sunfire Aegis if you roll that way and want even MORE CS control than your W provides.
Banshee's Veil
Another Warmog's Armor is always fun. or 6.

The skill sequence can vary and depends on your playstyle, which I would like to explain here. If you are jungling, getting Wraith's periodically while laning mid, or solo laning and farming CS, then max your Heart Zapper first. This makes your creep farm extremely powerful. Otherwise, grab it at level 2 and keep it there until level 10. This is also very helpful if the enemy is CC'ing you a lot.
Otherwise if you are aggressive Mundo, max your Blunt Force Trauma first. This skill is your bread and butter. It is the underestimated burst that picks kids up in the noob bus and takes them to class. You can eventually have this up constantly and have amazing AD throughout the game while building hardly any AD items. This is what makes Mundo such a beautiful hero; only one of his skills scales with AP or AD, and it scales by a low 40% of your AP (woww!) This allows mundo to build completely tankalicious and still have the damage you'd expect from him.

oh yeah, and Mundo's Passive, Goes Where He Pleases , provides him with a built in Regrowth Pendant for each 1k health he has. That's a pretty nice perk :)

Your Summoner spells vary depending on your playstyle. I prefer Exhaust . In my opinion, Exhaust is better for many reasons. It can be used to slow for 2.5 seconds to let your entire team burst somebody down. That is a lot more helpful than 100 true damage. If you are being chased, you can Exhaust them to escape. Losing a 1v1? Exhaust . Want first blood? Exhaust . About to get bursted down by akali's final Q R E combo? Exhaust takes all her damage away. You may choose to take Ignite instead for the antiheal and true damage, or maybe Teleport for the mobility as a tank. Cleanse is also very helpful and game-changing. This is your call, but you HAVE TO TAKE Flash. you should always take flash. with every champion. it makes plays, saves your *** from violation, and is extremely fun to use if you are good with it.

Pros:
Resurrection to full HP and, if played correctly, unkillable.
Built in Tenacity (but not exactly)
Built in Regrowth Pendant for each 1k health he has from passive.
Underestimated burst from his e
Massive health Regen
Did I mention he resurrects to full HP?!
#JesusStatus

Cons:
People...won't like vsing you?
Not a lot of skins if you're a skin *****