Iote's Shield

Iote's Shield is an item in the Fire Emblem series that first appeared in Fire Emblem: Mystery of the Emblem. It protects the wielder from effective-against attacks. In Fire Emblem Awakening, it was an item that granted a skill of the same name and effect.

Fire Emblem: Mystery of the Emblem
In its first incarnation, the Iote's Shield negates all Weapon effectiveness to its user. This includes both Bows for Flying units but, can also be used for other units' weaknesses.

Shadow Dragon and New Mystery
In the remakes, they tone down the effects of Iote's Shield. Instead of preventing all Weapon effectiveness, it only negates the effective bonus of Wind magic and Bows.

Fire Emblem Awakening
The Iote's Shield is the reward for completing the DLC chapter, Smash Brethren 3. When used, it grants the skill Iote's Shield. The skill grants the same effects as Iote's Shield as it did in Shadow Dragon

Fire Emblem Heroes
Iote's Shield is a Type A Passive skill that neutralizes the effective against Fliers bonuses like with weapons like Bows and the Tome Excalibur

As an Item
In Mystery of the Emblem, it is dropped by Michalis in Chapter 17. It can also be picked up in Book 2: Chapter 3 as a hidden treasure.

In Fire Emblem: Shadow Dragon, it is dropped by Michalis in Chapter 22.

In Fire Emblem: New Mystery of the Emblem, it can be found in a treasure chest in Chapter 13x.

In Fire Emblem Awakening, it is obtained as a reward for completing Xenologue 9, Smash Brethren 3.

As a Skill
In Fire Emblem Awakening, every character can learn the skill if they use the item. It cannot be inherited though so the children must use the item themselves.

In Fire Emblem Heroes, the skill is naturally exclusive to Michalis but, it was available as a Sacred Seal in the January Tempest Trials though it can crafted using Verdant Badges and Sacred Coins.

Trivia

 * The Delphi Shield in Binding Blade and the Fili Shield in Sacred Stones have the same function as the remake's Iote's Shield
 * In Fire Emblem Heroes, the game has Grani's Shield and Svalinn Shield which are effectively the same skill as Iote's but, for cavalry and armored units respectively.