In pubs, you'll probably find him in a Dual Hard Lane, or even solo (which he can do decently well, due to getting a higher HP pool from Decay and being able to last hit with it to some extent).
The problem you'll have is that he's mediocre late game. He's extremely aggressive early to mid game though. And you'll want to take advantage of that. Knowing when a hero peaks is part of playing them (e.g. you can initiate with Crystal Maiden before level 6-8, but after that she'll get torn apart).
- Get One Level of Decay (unless you are specifically doing something else, then you might want level 1 Tombstone)
- Max out Tombstone next
- Max out Soul Rip after that
- Take Ult when you can
- Finish off Decay after that
- Lastly, get stats
- Getting additional levels of Decay:
- Reduces Cooldown
- Increases Mana Cost
- Increases Duration
- Increases Damage
- Why I leave this at level 1:
- You have absolute shit for a mana pool - each level increases cost by 20 mana
- Tombstone is better
- Soul Rip is more versatile (damage or healing)
The one exception I've heard of is taking Undying solo mid against a STR hero like Pudge because then he cannot be aggressive against you (i.e. Rot will kill him before you), you'll also probably have a bottle to resolve your mana issues.
Some players like going multiple levels of Decay early. Could just be a personal preference thing. Perhaps they're with a KotL, who can give them a lot of mana for it. I just find it less useful overall (Soul Rip can heal your Tombstone if it gets focused).
- You need to max Tombstone early, it is more or less why you pick Undying in a match
- Tombstone will stop pushes, or allow you to be aggressive and commit to a fight
- Tombstone is very hard to deal with early on because it has too much HP for the enemy to get rid of quickly
- Soul Rip combos well with Tombstone for damage or healing an ally that is about to die
Undying will change drastically based on who you're up against. He's a Utility hero at heart and builds what the team needs most.
You're also going to want some Tankiness because you ult provides damage amplification based on how close you are to your enemies (750 range max). They're also slowed by a static 9%.
This item deserves its own section. TP Scroll are huge for Undying (especially in pubs where you probably aren't aggressive trilaning). Undying is great at turning ganks around, making a dive into a double kill, etc. Get TP Scrolls. Watch the map. You should probably pick one up by 4 minutes into the game or so, as that's when shit usually starts happening.
- Arcane Boots - this is going to be your choice most of the time. You need the mana, most teammates need mana. The only time this might not be applicable is if you have a KotL and you're the only other mana dependent hero.
- Power Treads - this can be another good choice because it gives you some tankiness and more mana. Switching to different stats will help you cast more spells, take more damage, etc.
NOTE: You might not be the one getting support items, but these are good choices if you are
- Mekansm - more healing for the team, you'll also stay alive long enough to cast it in a fight
- Pipe - more tankiness against spells, survivability, etc.
- Sheep Stick - you can use the mana/INT, the regen, and stats. It also helps you shut down enemy carries.
- Heaven's Halberd - the STR and Evasion benefit you so you can provide Damage Amp more effectively. The Disarm is huge against right click heroes like Faceless Void, Phantom Assassin, Ursa, etc.
- Vlad's - if you have a melee carry, you might consider picking this up for them and being somethng of an aura packmule for the team. You have high survivability
- Drums - same as Vlads more or less. Good item too if you're behind a bit because all the stats benefit you.
- Shiva's Guard - more slow is good, the armor is good, the INT/mana is good, and the aura is excellent on a tanky hero
- Necro Book - excellent for pushing, excellent for chasing, really good against heroes that might accidentally kill the melee creep (does 200/400/600 PURE damage), the stats work well for Undying too
- Assault Curiass - armor is good, the aura is good, the attack speed on you is kind of meh (depends on the amount of strength stolen)
- Also, see Heaven's Halberd in the Support Section, as it can be Utility as well.
- Heart - good for survivability if that's what you need most
- Armlet - good if you're the primary farmer and get it early for midgame dominance. You're sustained by Soul Rip and the healing from kills in your Ult Form
- BKB - you might need this to stay alive in fights. Being able to do Decays during fights is crucial to your survival. You can't do that while stun locked.
Things I don't really see a purpose in getting:
- Aghs - I don't think it buffs you enough. The stats and nice, but doesn't seem very necessary in comparison to getting items with higher utility
- Vanguard - You get plenty of HP from Decay. It just feels like a waste to me. Grab the Vitality booster and get a heart instead.
Using your Skills
- Use it pretty frequently in lane, not so much that you can't Tombstone though
- Try to only use it if you can get 2+ heroes in the AoE, or at very least the carry (so they're afraid to last hit)
- Use it as soon as it's off cooldown during fights - this is how you survive and get huge
- Only drop this once your team has committed to a fight
- Try to place this so your opponent is stuck in this range as long as possible (generally cutting off the escape route)
- Defend the shit out of this
- You can Soul Rip your Tombstone to heal it
- You can use Tombstone defensively too - once the enemy has committed to taking a tower or something
- If you're not sure whether to put it down, DON'T, the cooldown is too long to waste a tombstone. Good opponents will try to bait it out. That's why it's better as an aggressive skill. You KNOW when your team is going to commit. You don't know when the opponent will.
- You can use this to heal your Tombstone
- Use this to heal yourself or allies that are almost dead
- Use this to do crazy amounts of damage mid game to high priority opponents (e.g. annoying supports, carries), this works really well once you've spawned a few Zombies
- Again, don't use this until you commit to a fight
- The closer you are to an enemy, the more damage they take (i.e. the damage amplification aspect)
- If you're low HP, kill an enemy creep (or ideally a hero) to heal a percentage of their HP