Prevention effect

A prevention effect is a continuous effect that acts as a shield against damage. Most are specific and limited as to which damage interactions would be prevented, such as "prevent all damage that would be dealt to you this turn by a source of your choice". Prevention effects behave similarly to replacement effects: they replace the damage part of an event with "nothing" or with a reduced amount of damage.
==Description==
Prevention effects can come from one-time spells and effects, or from static abilities. They are usually limited in several ways, such as by which recipients of damage are protected (e.g. "to you", "to [this creature]", "to a creature you control"), which sources of damage the prevention works against, a duration or specific event the effect applies (e.g. "this turn" or "the next time damage would be dealt"), and how much damage in any damaging event is prevented (e.g. "the next 3 damage"). Some effects say damage "can't be prevented", which may apply to a single damage event or to all damage within a specified duration. A few cards have abilities that trigger when damage is prevented.
Damage prevention prevents the direct results of damage, which are usually accumulation of damage that may be lethal damage, loss of life, or loyalty counters. It also includes those from Infect, Wither, Toxic, Lifelink, and can prevent effects that would've replaced the damage with something else. It stops or reduces indirect consequences such as abilities that trigger from damage being dealt as well. If damage is reduced to 0 by prevention, those abilities will not trigger at all.
Some effects cause loss of life without "damage". These are unaffected by damage prevention.
==Related mechanics==
Damage prevention is closely related to replacement effects and follows most of the same rules. If multiple prevention or replacement effects would apply to the same player or permanent, that player or controller chooses the order they're applied; sometimes applying one effect first may make others inapplicable. However, replacement effects that replace damage with something else without using the word "prevent" aren't prevention effects, and ignore "can't be prevented" clauses.
Redirection effects are similar but distinct from prevention: damage that would've been dealt to a permanent or player is dealt somewhere else instead. These are usually replacement effects and do not use the word "prevent", so "can't be prevented" effects don't apply. Prevention effects can behave almost like redirection, using an "if damage is prevented this way" triggered ability to deal the prevented damage somewhere else (e.g. <c>Phyrexian Vindicator</c>).
Shield counter rules define a static prevention effect combined with indestructibility for any permanent the counter is on, but the counter is removed as the prevention happens. It's still removed even if it fails to prevent damage due to "can't be prevented" effects.
All prevention effects pertain to damage itself rather than related game events or conditions. 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. Preventing damage from a spell, ability, or combat doesn't counter the spell, affect the source of the ability, or prevent a creature from attacking or blocking. 
==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 <c>Samite Healer</c>. 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. The last partial prevention effect was printed in 2021 () and there have been no others since 2018 in Dominaria.
The Fog style of prevention of all combat damage, nicknamed after the Alpha card which began the trend, has continued to appear on new or reprinted cards for over 25 years now, for instance <c>Root Snare</c> in Core Set 2019. Gaseous form is a similar style of two-way preventing effect that has continued to see print. Self-affecting prevention effects are often used by planeswalkers who can become creatures, such as <c>Gideon Jura</c>. A more modern example, a specialized card with built-in limitations, is <c>Nine Lives</c>.
==Variations using counters==
Permanents with damage prevention can be kept balanced with counters that must be removed each time damage is prevented, for example <c>Spore Flower</c>, <c>Fylgja</c>, or <c>Defender en-Vec</c>. These may have additional abilities to replenish the counters. Shield counters have built-in rules prevent damage and destruction while removing themselves automatically.
A specific variation of this is for low-toughness creatures to have counters to prevent damage to themselves, as with <c>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. This essentially turns "taking damage" into "removal of counters"; unlike damage, this affects both power and toughness and is permanent. This ability first appeared on in Alpha, and it is still associated with Hydras, although not limited to them. As with other permanents, there is often a way to replenish counters. Thus, it's somewhat of a precursor to the planeswalker loyalty system.
Depending on the ability phrasing of a creature - like <c>Protean Hydra</c> or <c>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. <c>Undergrowth Champion</c> from Battle for Zendikar was the first creature printed with a specific clause to prevent this situation, although 's own revised oracle text uses the same phrasing.
One variant is the "Phantom ability", named for a series of creatures in the Judgement set named "Phantom [creature]", such as <c>Phantom Nishoba</c>. These creatures prevent all damage to themselves at the cost of a single counter per damaging event.
A rarer 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 <c>Phyrexian Hydra</c> and <c>Lichenthrope</c>. This is rare not from complexity but from the lack of design space for -1/-1 counters. One other variant reverses the effect entirely, by gaining +1/+1 counters with damage taken, as with <c>Phytohydra</c> and <c>Ironscale Hydra</c>, which naturally causes the creature's stats to increase rather than trend toward 0 toughness.
==Rules==
==References==