Osty guide

Summon adds health and max health to Osty. Summon can be thought of the same as blocking, but it doesn't go away at the end of the turn (like the Barricade buff). This allows you to store up extra defenses when you're not being attacked (and makes your Unleash card stronger)

Healing Osty will only help if he's not already at max health. When Osty dies, his max health is reset to zero until you summon again

Enemy attacks will reduce your block first before going to Osty's health. There are a few kinds of damage that can be blocked, but go straight to the necrobinder's health (bypassing Osty) if not blocked, like damage from burns and decays at end of turn, or thorns damage from enemies

Osty can be useful in doom builds to tank damage from the enemy's final hit before it gets doomed

If a card says "deal damage", then the necrobinder attacks. If it says "Osty deals damage", Osty attacks

Osty's attacks are unaffected by most buffs or debuffs on the necrobinder, like strength / negative strength, shrink, or weak. But enemies who enchant cards in your hand (like doubling the cost of attacks) can still affect Osty's attacks. Osty still benefits from debuffs on the enemy, like vulnerable

If Osty is dead during your turn (usually doesn't happen unless you sacrifice him), he can't attack

There are several cards that synergize with Osty's attacks. This is an archetype you can build around to do massive damage; it's super effective. You can combine these with soul cards to help you draw into more attacks each turn. List follows:

Squeeze does more damage for having other osty attacks in your deck

Calcify buffs osty attacks

Sic 'Em summons when you play other osty attacks

Rattle hits more times for each osty attack you've played

Flatten is free to play if you've played an osty attack this turn