Heals adjacent ally 10 HP. Unit loses 10HP (but cannot reach 0 this way)
Ardent Sacrifice can be considered an Alternate version of Reciprocal Aid, as it uses each hero's HP pool to provide healing. However, it is limited to transferring only 10 HP. While Reciprocal Aid has more raw healing potential, Ardent Sacrifice has the advantage of being much more consistent. Only 10 HP is transferred and while that might seem small, it can be enough to swing many duels in the player's favor. Within the Arena not every unit is equally valuable in every match, and Ardent Sacrifice is a great way to ensure a unit can always contribute to the battle, even if they don't fight directly. Ardent Sacrifice is most commonly not used for the heal: It's used for the damage, which can help trigger skills reliant on HP thresholds.
The healing effect of this skill has the potential to be useful in nearly any game. Combined with self-healing skills (Such as Recover Ring, Renewal or a Falchion) A unit can potentially use Ardent Sacrifice many times while restoring their own HP. If aiming for a pseudo-healer set however, it is often better to use Reciprocal Aid as it provides a much stronger potential heal. Ardent Sacrifice's biggest detriment is actually one of it's greatest strengths, as the HP loss can be used to trigger threshold skills such as Wrath, Vantage and Desperation. These powerful skills can make a unit unstoppable on the field, and Ardent Sacrifice can be used to circumvent their biggest weakness: the HP threshold. However, a unit wishing to trigger a 75% threshold ability with one use of Ardent Sacrifice must have 40 or less HP (40 – 10 is 30, which is 75% of their max HP). This means that a unit using this combo can sometimes benefit from a HP stat bane, if they would normally have more than 40 HP.
Both benefits to this skill have a drawback. The heal itself is not a true heal, as it weakens another unit who would otherwise be healthy. And the drawback to using the HP loss to activate skills is that it requires a unit to have lost HP. Ardent Sacrifice cannot be used on a unit with full HP: The unit receiving the heal must have already lost HP. This means it can't be used at the start of a round, some combat must take place before it can be used.