Prevention effect

A prevention effect is a continuous effect that acts as a shield against damage. Most such effects apply to some combination of specific sources, recipients or quantities of damage, as well as time periods in which damage would be dealt. Prevention effects behave similarly to replacement effects which replace the damage part of an event with "nothing" or with a reduced amount of damage.

Mechanics
All prevention effects pertain to damage rather than other game events that may be stopped from happening. For instance, countering a spell, removing a creature from combat, or a static ability that states a certain kind of event "can't happen" are not prevention effects. Some of these abilities may employ the similar language or rules logic of replacement effects instead.

Prevention effects don't prevent the action that caused the damage from completing: damage-dealing spells are not countered, and creatures are still considered to have attacked even if the resulting combat damage is prevented. Damage prevention does prevent the direct results of damage, which are usually loss of life or loyalty counters, but may include others like those from Infect or Wither, and those caused by other replacement effects. Prevention also stops indirect consequences such as abilities that trigger from damage being dealt.

Some effects cause loss of life without "damage". These are unaffected by damage prevention.

History
In the early history of the game, damage prevention was somewhat common and was considered a special category of effect with its own place in the timing rules, together with damage redirection (which is now a subtype of replacement effect) and Regeneration. Creatures that tapped to prevent damage were perhaps the most common, such as the classic Samite Healer. R&D has stated it has scaled back on doing damage prevention, because it adds a lot of board complexity and slows the game down. It's primary in white. Dominaria's Healing Grace and Shield of the Realm marked the last partial prevention effects printed for several years until Valkmira, Protector's Shield in Kaldheim.

One style of prevention, nicknamed after Fog, the Alpha card which began the trend, has continued to appear on new or reprinted cards for over 25 years now, for instance Root Snare in Core Set 2019. Self-affecting prevention effects are often used by planeswalkers who can become creatures, such as Gideon Jura, for many years. A more modern example, a specialized card with built-in limitations, is Nine Lives.

Preventing damage by removing counters
One way damage prevention is kept balanced is through the use of counters on permanents that must be removed for each point of damage prevention, possibly with the opportunity for replenishment of such counters, such as with Fylgja</c>, Ursine Fylgja</c>, or Palliation Accord</c>.

A specific variation of this is for low-toughness creatures to have counters to prevent damage to themselves, such as with Rasputin Dreamweaver</c>. This is especially common specifically with 0/0 creatures that come into play with +1/+1 counters which are then removed to prevent damage, essentially "taking damage in the form of removal of counters", turning damage into something that affects both power and toughness, and permanently rather than temporarily. This ability first appeared on Alpha's Rock Hydra</c>, and it is still associated with Hydras, although not limited to them. As with other permanents, there is often a way to replenish counters.

Depending on the ability phrasing of a creature - like Protean Hydra</c> or Unbreathing Horde</c> - it may become "invulnerable" to damage if its toughness is modified to be non-zero without the use of counters, such as with an enchantment. This allows it to continue preventing all damage, but not losing toughness because it has no further counters to remove. Undergrowth Champion</c> from Battle for Zendikar was the first creature printed with a specific clause to prevent this situation, although Rock Hydra's own revised oracle text has a similar interpretation of the original Alpha edition phrasing.

One variant is the "Phantom ability", named for a series of creatures in the Judgement set named "Phantom [creature]", such as Phantom Nishoba</c>. These creatures prevent all damage to themselves at the cost of a single counter per damaging event.

A more rare variant is the inversion of the role of counters and base power/toughness by putting -1/-1 counters onto a creature with high statistics, rather removing counters from one with low toughness, as with Phyrexian Hydra</c> and Lichenthrope</c>. One other reversing the usual effect entirely, by gaining +1/+1 counters with damage taken (instead of losing them), as with Phytohydra</c> and Ironscale Hydra</c>

Rules
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