Stoneheart Units are special Units that you can train after unlocking Stoneheart Castle and building the Stoneheart Tower. There are 4 Units in total: Valkyra (Infantry), Cavalier (Cavalry), Sorcerer (Occult), and Drake (Bestiary).
Stoneheart Units are suitable for use in both offensive and defensive engagements, and will gain a 20% stat advantage in battles against non-Stoneheart Units. Their basic Offense and Defense statistics are equal, although the Masteries for each Unit type allow you to tweak their offensive or defensive statistics according to your strategy.
Stoneheart Units can be trained at Stoneheart Tower, but you must discover the Stoneheart Mastery for each Unit before you can train them. Training Stoneheart Units requires both Adamantine and Soulstones. However, you can also win Units as a Reward for completing Stoneheart Trials and Stoneheart Saga Trials.
You can boost your Stoneheart Unit Training Queue using both Regular Boosts and Sapphires.
If your Stoneheart Units are killed in battle, you are able to revive a certain percentage of them in the Infirmary or in the “Healer” tab at the Black Market.
The percentage of Stoneheart Units that can be revived using Sapphires depends on the “Infirmary Rate” Mastery for each Unit and the Level of your Infirmary. You can also revive a percentage of Stoneheart Units (those who fell defending your Castle) for free - these will appear in the “Free Revival” section in the “Stoneheart” tab of the Infirmary.