Source: Storm King's Thunder

Wondrous item, very rare (requires attunement)

This appears to be a simple ingot of iron ore, about a foot long and a few inches across. Inspection of its surface reveals the faint, silvery outline of the skold (shield) rune. The ingot has the following properties, which work only while it's on your person.

Runic Shield. You have a +1 bonus to AC.

Shield Bond. As a bonus action, choose a creature that you can see within 30 feet of you, other than yourself. Until the end of your next turn, any damage the target takes is reduced to l, but you take half the damage prevented in this way. The damage you take can't be reduced in any way. Once you use this property, you can't use it again until you finish a short or long rest.

Shield Ward. You can transfer the ingot's magic to a nonmagical item — a shield or a two-handed melee weapon — by tracing the skold rune there with your finger. The transfer takes 8 hours of work that requires the two items to be within 5 feet of each other. At the end, the ingot is destroyed, and the rune appears in silver on the chosen item, which gains a benefit based on its form:

• Shield. The shield is now a rare magic item that requires attunement. Its magic gives you a + 1 bonus to AC, and the first time after each of your long rests that damage reduces you to 0 hit points, you are instead reduced to 1 hit point. You must be wielding the shield to gain these benefits.
• Weapon. The weapon is now an uncommon magic weapon. It grants you a +1 bonus to AC while you're holding it.