Horde Mode is a cooperative PvE (Players vs Enemies) mod for Dota 2 with the goal of allowing players to battle massive amounts of creeps and bosses while preserving the feel of vanilla (unmodified) play.

There are many changes that were made to the vanilla game to allow every hero and item to remain useful at some point in play.

In this guide I will outline the basics of play, skim through changes made to the map, heroes, items, the differences in roles, and basic strategies to survive the onslaught.

You, the Radiant defenders, will be pitted against creeps and bosses, and just the four of you (five if you put on the training wheels) will defend. The Dire will come in waves and try to destroy your ancient; You will need to destroy their ancient to win.

The map in Horde Mode is the same across all difficulties; they differ only in the Dire Horde's strength. There are a few key changes to the map, not too much, though;

• The top lane has two camps that auto stack every minute
  
• The Roshan Pit now contains an inventively named and more powerful boss
  
• Your creeps do not march beyond your towers
  
• Dire creeps have a chance to drop gold bags that give all Radiant heroes a small bounty
  
• Either team gains a global buff from pushing down the others' towers and barracks

This would be a massive section if I listed every change made to every hero, because currently every hero except Meepo is present in the mode, and the heroes less suited to PvE were buffed but not entirely changed, as the goal of the developers was to create a co-op game that was similar to the vanilla experience. Suffice to say every hero has been edited at least slightly if they weren't good.

The items in Dota 2 were not created for a PvE mode, and the developers have balanced certain items.

Almost all consumables (like mango, potions, etc.) were cut in price, and some were added or changed. To keep it brief, here are the items that were added or changed:

• Quelling Axe, early cleave power, also Quelling Blade cleaves, Dragon Javelins are ranged AoE, Skadi is magical AoE
  
• Wand is now granted charges by last hit, key item here!
  
• Dagon has AoE and can combine with Bloodstone
  
• Vlads can combine with AC, SnY with Atos, Phase Boots with Desolator
  
• Battle Fury, Power Treads, Radiance, and Divine Rapier have upgrades

Full changes at http://steamcommunity.com/workshop/filedetails/discussion/472597026/371918937281233289/

Roles are largely the same, except there is no way to be an initiator or ganker, since these are roles specifically for killing heroes.

There is a new role, the boss-hunter, these are heroes with strong skills for killing bosses and large enemies. A good example is Lifestealer, as he drains a percentage of health every attack, along with most other % damage heroes, and heroes with strong disables like Crystal Maiden.

Many heroes now can partially be nukers, as the weakest heroes were often give AoE damage to compensate, such as Skywrath Mage's Arcane Bolt being granted an area of effect.

Nukers and other casters often are strongest at the very beginning and the middle of the game, being able to farm last hits easier than other heroes.

Carries, as in vanilla, still scale better at the end.

Melee heroes benefit from the Stout Shield and the Poor Man's Shield in the beginning of the game and into the midgame due to the fact that it blocks a moderate amount of damage from every hit taken, and you are constantly hit by Dire creeps.

Cleave is essential for melees as you need to hit multiple enemies in order to clear waves.

Ranged heroes will probably want Dragon Javelins or Mjolnir, in order to have their own form of AoE damage.

Likewise, in order to make a caster hero have AoE, there is the Horde Dagon.

The Heart of Tarrasque is the ultimate in survivability, as there are no enemy heroes in Horde mode, you always get the regeneration.

Since the Dire get buffs from destroying your buildings, defend them at all costs.

Even though the mod is PvE, picking the correct heroes to form a cohesive team is important.
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credits:
Nye for creating the game
Diellan for recently taking the torch in development
TheLiquidPepper and Siege for helping me learn this stuff myself.