Black Mantle is Iwodach's utmost curse, allowing a battle witch to negate the effect of heals on the affected victim in a similar manner to unholy damage's Mantled status effect. Unlike the status effect, which must be built up by repeatedly striking the opponent with unholy damage and which can be mitigated by unholy resistance, Black Mantle directly shrouds the victim with opposite polarization the instant it is cast. Furthermore, extremely powerful Black Mantle curses - typically from Xaerdun witches using Unhallowed Runes to boost curse effectiveness - can even cause heals to harm the victim instead, something unholy's status effect can never do.
If a heal would harm a Black Mantled victim, that heal becomes an attack and takes on safety precautions and combat restrictions the same as any attack spell. I.e. You can't be killed by heal mules just because you're Black Mantled. However, they won't be able to cast heal on you at all. Conversely, players that can fight you are perfectly capable of Black Mantling you and then 'attacking' you with Major Heal.
Hex effectiveness determines the damage a reversed heal will do. At 100% Black Mantle (a normal curse), heals do nothing. At 133% Black Mantle (boosted by Unhallowed Runes), the heal will harm you for a third of the amount it would have healed. E.g. a heal for 30 instead damages you for 10. Effectiveness percentages higher than that are not currently attainable by players, but monsters that apply extremely powerful Hexes can cause heals to do extreme amounts of damage. Be wary. There are tales of demonic spirits that apply Black Mantles so strong that the heals they cast on you become deadlier than any lightning bolt.
If you're killed by reversed heals, the server will see a special death message.
If you kill yourself with a reversed heal, the server will see a different special death message.