Initiating

Initiating is the capability of Heroes to cripple a group of enemies before a clash to gain an upper hand. The concept of initiating is central to success at the mid-and late game phase of any game of dota, and mainly deals with the art of starting a teamfight in such a way that your team already is advantaged when the fight begins. In a team without heroes well suited to this task, each Teamfight could become a messy affair, or your team runs the risk of always getting attacked first.

There are several ways to accomplish this: Either catch the other team while they are badly positioned, at a time when your own teammates are ready to pounce on them, or isolate and immediately kill one enemy, then mop up the rest when they clump up to try to defend him. There are a hundred ways to handle a good initiation, but often, the ingredient is a hero with a Blink Dagger and a game-changing Ultimate, a warded map, and a very good sense of intuition and map awareness.

Being the Initiator means that you are basically in charge of aggression for your team. However, some heroes who are commonly labelled as Initiators (Sand King, Earthshaker, Enigma) are equally good at immediately winning a team fight that is already underway, because the other team might be loathe to clump themselves together under the threat of huge aoe damage until they have an opportunity to actually attack the other team.

Heroes
Initiators are the most important driving force in winning team clashes. Most if not all Initiators have (an) area skill that either delivers superb damage or infuriating crowd control status effects to the whole of the enemy team. Every Initiator also has their own different role, usually as a Tank, Support, or Roamer. Some Initiators, however, focus more on decimating the most important target in the team that might ruin the clash for them such as a Carry.

Be reminded that playing as an Initiator will most likely require you to expose yourself. With that said, players using Initiators should not exhibit fear else it'd just make their role pointless.

Special mention Initiators are Tidehunter,  Earthshaker,  Sand King,  Clockwerk,  Axe,  Enigma, and  Vengeful Spirit.

Items
Items really depend on what type of Initiator one could be. If their initiating skill requires them to be close such as Earthshaker's Echo Slam and Tidehunter's Ravage, an item such as Blink Dagger, Force Staff, or even Shadow Blade can really ease the job. If their initiating skill requires them to be uninterrupted throughout the battle such as Sand King's Epicenter or Crystal Maiden's Freezing Field, Black King Bar allows them to to remain active throughout the clash for them to deliver the most out of everything. Initiators also need to disable the most amount of enemies at one time, so Eul's Scepter of Divinity and Scythe of Vyse are also great items. Area debuff items such as Shiva's Guard and Assault Cuirass also greatly help. Initiators also make great carriers of area support items such as Mekansm and Pipe of Insight. Items that increase survivability such as Heart of Tarrasque and Blade Mail also work.