Luna (also known as Pierce) is a skill in the Fire Emblem series that upon activation, ignore the defense of the opponent during the attack. Later versions of the skill only half the enemy's defense and resistance. It was introduced in Fire Emblem: Genealogy of the Holy War.
Genealogy of Holy War and Thracia 776
The skill negates the target's Defense and the move is always successful. The activation rate is the user's Skill.
The Sacred Stones
The skill is called Pierce here and i does negate the opponents defense. The trigger is based on the user's level though and it seems the move can miss.
Path of Radiance
This is the first instance of the skill becoming its modern version. it halves enemy Defense and Resistance, but as a result, it cannot perform a critical.
The skill is its strongest here. It triples strength and negates enemy's defense but, the skill does not work with thrown weapons and has an activation rate of half the user's skill.
The skill halves the enemy's Defense and Resistance. It triggers based on Skill. This is how the skill behaves in all games since Awakening.
Echoes: Shadows of Valentia
In Fire Emblem Echoes: Shadows of Valentia, the skill is called Roundhouse where its the effect of Silver Sword. Effectively, it has the same effect with it ignoring half of the enemy's defense or resistance but, the skill requires 20 skill points and uses 7 HP. It also adds 4 Mt with +20 to Crit, +20 to hit and +20 to Evade.
Fire Emblem Heroes
In The Sacred Stones, it is the Class skill of Wyvern Knights. In Path of Radiance, the skill is the Occult skill of Halberdier and General. In Radiant Dawn, it is the skill of Marshalls. In Awakening and Fates, the skill comes to Great Knights starting at level 5. In Shadows of Valentia, the skill can be learned from the Silver Sword.
Fire Emblem Heroes
The following characters learn this automatically
This is a Lunatic+ version of the skill. It only appears on enemies in Fire Emblem Awakening and the skill now always activates.
This is a stronger version of Luna, bypassing 80% of the target's Defense and Resistance when attacking. It is exclusive to the Black Knight. It is likely a take on the Eclipse skill.