Avalon is probably the most powerful of the three base heroes, as he provides the most support to your army. He can deploy 3 legions of units compared to the normal 2. He is a powerhouse of a support unit which can really change the outcome of any battle, ideally you should use a human army if you plan to use Avalon as your primary hero, he also does well with dwarf humanoid units.

Arrow Volley

Avalon targets a volley of 80 flaming arrows on the battlefield. Enemies struck take X points of burning damage. Troops that die from the burning damage will explode Bonus based on Might.
This ability allows you to target snipe out important targets like enemy archers or even heroes such as Vega. Using this ability in such a way will change the outcomes of a battle.

Summon Archers (costs 1 Energy)

Avalon summons a group of Archers to assist in battle for 30 seconds. The overall Power of the summoned Archers cannot exceed X. (summon silver archers at rank 4, gold at rank 8)
This ability allows you to summon two extra legions of archers and place them anywhere in range, this is incredibly powerful, as you can just summon archers into the enemy backline and they will just destroy everything. Try and use this skill sparingly as it does cost one energy to use and energy is your most valuable resource.

War Horn

When he takes the battlefield, Avalon blows his War Horn, increasing the attack speed of nearby allies by X% for 8 seconds.
This ability is very nice if you are able to take advantage of it, try and place Avalon in the center where he can buff 2-5 legions of troops with the war horn, if you place him and your troops properly you can double your damage output for initial start of a fight which is critical to starting a landslide victory and chaining morale boosts.

Archer Mastery

Avalon is skilled at leading archers into battle. When he is on the battlefield, all allied Archers, Longbowmen, and Feather bows have their damage and max HP increased by X%. (32% at 8/7)
This ability makes archers an already high damage per second unit even stronger, it is actually crazy how high archers damage is when you max this and War horn, things melt.

Ardent Aura

Avalon leads an extremely disciplined army. When he is on the battlefield, all allied humanoid units have their max hit points increased by X%. (32% at 8/7)
This ability is outstanding as it allows your squishy humanoid units to gain a large percentage of health which is very much appreciated since human units lack a tanky frontline unit that can take a hit.

Natural Leader

As Prince of the Silverwings, Avalon is beloved by his troops. They are heartened when he takes the field, allowing him to deploy 1 additional troop. Also increases your bronze supply cap by 40+40/level, which in turn raises your Silver and Gold supply caps. (100 bronze = 5 gold supply)
Get this skill one rank is all you need but this skill is probably one of if not the most powerful skill in the game, allowing you to field one extra legion onto the battlefield, having 5 legions of archers instead of 4 while still maintaining your frontline or having an extra legion of priests for healing is amazing. Maxing this skill also provides you with 8 extra gold supply which is half a pack of gold priests, or 8 more gold archers this is huge, especially with the new academy research that increases legion sizes this 8 supply will be needed in the end game.

Building Avalon

How you build Avalon depends on what race you are so I will list three builds for Avalon

Human Based Avalon

When using Avalon with a human army you want to focus on archers, so get archer mastery as high as you can and get summon archer as high as you can. Get one point in natural leader for the extra legion deployment and after that I would spend extra points into ardent aura to provide your whole army with some extra health. If you still have ability points left over go into Arrow Volley, leaving War Horn out of the picture, as it is nice but doesn’t provide the strategic bonus of a well-aimed arrow volley on a squishy target.

Lich Based Avalon

Get one point in natural leader and max out Summon archers, then the rest into arrow volley everything else is wasted on lich unless you want to go with War Horn and have him be an attack speed buff bot for a few Spiders (two spiders max unless you want to place your units in a none standard way which could be dangerous)

Dwarf Based Avalon

Now depending on what army type you go as dwarf changes Avalon, if you go with a humanoid army build him the same as you would as Human based Avalon, if you are using mechs as your army build him as you would build Lich Based Avalon.

Equipping Avalon

Avalon is a hero based around supporting an army so his primary stat you want him to have is Command. He is also a melee physical attacker so you also want to give him some Might and Stamina as the might boosts his arrow volley and provides physical damage reduction, while stamina gives him a larger health pool.

An ideal equipment setup besides full legendary items for Avalon would be as follows:

Weapon: Radiant Edge (+3 might +1 command)
Banner: Imperial Banner (+4 command +1 to Archer Mastery)
Helm: Aylan’s Crown (+8/9 command +3 stamina) this item is acquired from the last dragon boss
Armor: Radiant Breastplate (+1 command +3 stamina)
Relic 1: Radiant Ring (+1 might +3 command)
Relic 2: Golden Goblet (+4 command +1 to Ardent Aura)
Having 3/6 of the Radiant items equipped provides +8 command to that hero, which is equal to a legendary item in stats alone, so having 3/6 equipped on Avalon is very good.

If you choose Avalon your default second hero you will get is Avril a few minutes into the game.