Voil Guide, from a BattleGroup perspective disclaimer: this is just scratching the surface, and I'm less experienced as a player than as a collector.
Advantages: flying, multiple damage types, can work as a themed battlegroup
Disadvantages: limited options like any theme,
Voil Champs
Lance Admiral Sceian: 107 nora, Rise of Serkan exotic.
advantages: high damage, good defense, boosts other units
disadvantages: high nora cost, FF only, unequippable
Voil Bomber: 78 nora, Drums of War common
adv: bombs have knockback, can learn mobility, can shatter equipment
dis: physical damage only, below average damage
Voil Courier: 72 nora, runeforge only
advantages: deflect, range 1-3, good upgrades
disadvantages: hard to obtain, limit 1 per BG
Voil Frenzy: 72 nora, Skeezick Rebellion common
advantages: high damage potential, high speed
disadvantages: works best when injured
Voil Imperialist: 74 nora, Maljaran War common
adv: Iron Will for all, strong defense, boosts voils, cheaper than Sceian, range 1-2
dis: speed 5
Voil King Ruubgaal: 97 nora, Heroes of Maljara exotic
adv: greatly boosts lightning BGs, has Boon of the Undead
dis: requires a SP/X or FW/X split battlegroup, costs as much as it's worth
Voil Lancer: 66 nora, Shattered Peaks common
adv: cheap, can ignore defense, never misses
dis: melee only, low hp, below average damage
Voil Lifestealer: 75 nora, Path to Conquest common
adv: can heal self, has mobility, high speed
dis: melee only, many vulnerabilities
Voil Queen: 74 nora, Shattered Peaks rare
adv: summons bats, can learn Surge: Voil
dis: melee only, bats have at most 10 hp
Voil Screecher: 68 nora, Shattered Peaks common
adv: good damage, good range, alternate damage type, can dodge
dis: below average hp, average speed
Voil Sky Captain: 73 nora, Dire Covenant common
adv: great against non-fliers, can learn assault and/or channel speed
dis: reduces allies' defense, no ranged ability
Voil Sorcerer: 80 nora, Grimlic's Descent rare
adv: good damage potential (surge: voil), good range, can explode (death nova)
dis: high cost, requires melee support
Voil Stormflyer: 90 nora, Broodcall rare
adv: heals quickly, strong electrical damage
dis: high cost, harder to get than many exotics
Voil Transporter: 63 nora, Drums of War uncommon
adv: teleports units for strategic placement, can learn mobility
dis: bad combat stats
Voil Windstriker: 82 nora, Ancient Awakenings uncommon
adv: kills enemy fliers, huge damage potential
dis: melee only, expensive
auxiliary champs
Bat Swam: 62 nora, Shattered Peaks rare
adv: much tougher than summoned bats, swarms itself
dis: melee only, 36 hp
Cavernkeeper: 38 nora, Path to Conquest common
adv: works as first aid station, immune to damage if next to a voil, is cheap
dis: low damage, easily killed without voil, is Voil Bound
Voil Progenitor: 115 nora, Nora Surge exotic
adv: high toughness, summons bats, works in multi-race SP
dis: high cost, works best in multi-race SP
Firebomber: 71 nora, Drums of War rare
adv: counts as a bat (and a moga), provides Fire damage, can lay traps
dis: vulnerable: Magic, below average damage
Spells, Relics, Equipment
This section needs some help. I don't use most of these personally. Most Voil decks use additional, non-voil runes.
Defend the Roost: 35 nora spell, Nora Surge common
teleports voil back to shrine and summons two small bats
Wings of Steel: 30 nora spell, Dire Covenant uncommon
takes away voils' flight, but boosts their defense
Voil Caverns: 40 nora relic, Broodcall common
grants Refuge to voils, unless it is destroyed
Voil King's Crypt: 60 nora relic, Heroes of Maljara uncommon
summons Bat Swarms, up to three times. see Bat Swarm champ
Sacred Peaks Amulet: 30 nora equipment, limited edition or rune forge
grants voils Battlemaster, grants moga or cyclops other abilities