Shadow/1.03

Shadows are advanced infantry units.

The upgrade from the Wraith, these are stronger in almost every way: +1 speed, +5 damage, +60 hp, +3 view and +4 combat.

They retain the ability to leech the experience from slain enemies, the vulnerability to all physical and resistance to all elemental. Unlike the Wraith, the Shadow gets a bonus +5 combat on top of the Undead's natural combat bonus at night, making the Shadow an even more powerful unit in night conditions. Heroes who have access to the Illusion, the Necromancy, or the Nature spell spheres can all turn day to night at will, although at different mana costs.

Although floating units, which entitles them to attack fliers, neither Wraith nor Shadow may cross bodies of water.

Dark Dwarves
Ancestors' Shadows are available at the Ancestral Hall once the Ancestral Spirit research is fulfilled, both through normal production, or instant summoning with the Summon Spirit building power, in emergency situations and at a greater cost. They can also be upgraded by the aforementioned building. Note that contrary to the Undead, Dark Dwarves can't morph (upgrade) their existing wraiths into shadows.

Shadows are also the Dark Dwarves' best unit to stuff their towers with in regards to improving the tower's stats, as they're the only advanced infantry unit Dark Dwarves get access to that can garrison.

Being available after only but one research once wraiths become themselves available (Ancestral Spirit 2), and given produced wraiths can't be upgraded into shadows afterwards, there are often no reasons for Dark Dwarves to invoke (produce) ancestors' wraiths. This could be obviously another matter if their early tower filler, the Firebomb, had a limited life span, or when crystal income is very low and Iron Golems can't summon extra firebombs.

Benefits from Weaponsmith, Armorer, Mithril, and Forgotten Power researches.

Undead
Shadows benefit from being produced by the Undead, because even with no hero, the Deathkeep may cast Dark Citadel, turning day to night very easily, though only from keep level 4 on. This effectively confers an additional +5 combat for a total of +9 combat over the wraith. Moreover, the Undead know how to upgrade their shadows (and wraiths).

These advantages make it very tempting to get shadows over wraiths. Yet the crystals required for the morph are also sorely needed for liches and magical upgrades later in the game. I believe the question of whether to use shadows over wraiths is directly tied to their role and special exp draining ability. Against races or heroes that grant bonus XP, there is a very strong incentive to get shadows. The greatly increased combat and hp scores helps to ensure that the shadow has a slight edge over even high XP units. The combat score helps it avoid getting destroyed by physical damage critical hits that would otherwise overcome a wraith. On the flip side, the combat score helps the shadow land critical hits of their own which is fantastic due to their cold damage type. Of course, the true power of the shadow can only unlocked at night, so if the opposing hero can turn night to day very efficiently, then the shadow loses much of its value. Then, you'd only want to morph into shadows when the wraith is at low life as this will heal them (depends on the patch).

The only time where it should almost always be beneficial to get shadows over wraiths is when you're fighting the fey. Unless you can rush them down, the fey often become a terrifying swarm of exceedingly fast units. You want all the speed you can get to rush into favourable fights and run away from bad ones, especially when their flying units abuse terrain. It's easier to retreat a smaller group of high hp shadows than a large group of wraiths due to pathfinding and stragglers. Those stragglers will be eaten alive by cute little faeries.

On the offence, the additional 5 damage is very useful since the fey has access to warding. 20 damage vs 15 damage (or 35 vs 30 with upgrades) may not seem like a big difference but when the enemy fey has even a modest investment in warding, the resistances will stack on top of the natural resistance bonuses from XP. You will want to hit as hard and as fast as possible before the quick-footed fey can regroup or abuse the terrain with their flying units. Landing those cold critical hits is even more important here as the fey usually have the advantage of mobility, yet another reason to prefer the shadow.

Benefits from Weaponsmith, Dark Mithril, Haunting, and Wailing researches.

Quotes

 * Ready: "Darkness arrives!"
 * Select: "What now?" "I am ready." "Always ready!"
 * Order: "Indeed." "'Tis a trifle." "I grow tired of this."
 * Deathblow: "Bow before your master."