Ultimate Heal is Kindness' 3rd Deltarune, 3rd Epic and 11th overall Spell added in Season 69.
Not to be confused with Heal, Ultimate Heal grants your monsters a global increase of +2 HP, making them tougher to kill. However, this has a major downside; the global buff also applies to enemy monsters.
By making all monsters healthier, using Ultimate Heal will make it harder for any monsters to be killed. This HP buff is most beneficial for tempo and aggressive decks, which want to keep their monsters alive for sustained pressure. Kindness' passive healing synergises well with the extra HP, making it easier to attack enemy monsters while keeping a decent HP pool. Like Force of Nature, combining this with Madjick can allow the added HP to be turned into ATK power.
Granting extra HP to enemy monsters can punish killing effects, Dust and anything with low ATK. On the other hand, losing control of the board becomes more punishing, as opponents can use the extra HP to build their own tempo.
Monsters that are Generated after casting Ultimate Heal will not gain the increased HP.