Concentration
Concentration is a limiting system that prevents a single combat unit from providing an unreasonable or game-breaking amount of duration-based effects (buffs or debuffs) simultaneously to themselves, their allies and their enemies. For buffs, this issue primarily affects games where a player can have large number of allies to buff (MMOs) or games where spells have reality-warping combo potential (DnD). Debuffs are not typically limited by concentration but players can find situtations where large number of enemies can be defeated using DoTs.
For the flip side where there are limits on how many effects a combat unit can recieve, see buff limit.
Games with no concentration mechanic and no balance issues
Plenty of games have been released over the years with no concentration system and no gameplay balance issues.
None of these games have ever implemented concentration or really had issues with buff stacking. However, they are single-player and small-party, and casting the buffs does take a significant chunk of the caster’s mana pool.
Game series that eventually implemented concentration due to balance issues
Everquest (no concentration) -> Everquest II (concentration)
The original Everquest had no concentration system for the caster - a character could cast buffs until they ran out of mana. Once cast, a buff has no further connection to the caster - they do not need to be grouped with the recipient, in the same zone as the recipient, be alive(!!) or even logged in to the game. This enabled players to provide buffing as a paid service - buffing newbies or entire groups with AC/HP bonuses, mana regen, melee haste, movement speed, resistances, stat buffs, etc. all for only 50pp. Casters with lots of buff spells could sit in town and make decent money just buffing, something that a non-caster like a Warrior or Rogue had no opportunity to do.
During the early days of Everquest, buffs were relatively short duration, usually lasting 30 minutes or less, which greatly limited the number of buffs a single caster could maintain with their (slow) mana regeneration. Also, group buffs could only target the caster’s own group, necessitating a “buffing dance” where the buffer would temporarily join a group, cast the buff, then drop and join another group. Buffing even small raids was a tedious affair with a short time limit - you had to finish the buffing process with enough time left on the buffs to complete the raid encounter.
As expansions were added, several key factors made buffing easier, but contributed to game balance issues:
- mana regeneration increased
- the base duration of buffs increased to 45 minutes or more
- characters could invest in abilities or gear that would further decrease the mana cost and increase the duration of their buffs
- many buffs became available in group form
- the ability to target a group different than your own was added, eliminating the “buffing dance”
- Mass Group Buff (MBG) allowed the caster to affect everyone in a huge radius, grouped/raided or not with a group buff spell once every hour
The upshot of this was to actually decrease the value of an individual buffer. Entire raids of 72 people could be mostly buffed in 15 seconds with only 2 or 3 members of each caster class using MBG. (Only once an hour, though. There have to be limits.) Groups could buy buffs in town with significant duration, instead of actually grouping with a buffer.
Personal note here about playing an enchanter>
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New page: Buff limits
For the recipient, there was a relatively small hard cap on the number of effects (either beneficial or detrimental) tracked on each unit, and a detrimental effect would overwrite a beneficial one if all the buff slots were filled. In normal group play reaching the buff limit was not usually a problem but could be in a large raid with lots of buff casters available or lots of detrimental effects being used by raid bosses. Also, DoTs counted in the effect count, and having several DoT-based classes could quickly use up all of the effect slots allowed on the enemy. It’s unclear to me if this is just a programming limitation to avoid performace issues, or a gameplay limitation to make players choose which buffs to use on themselves and which debuffs/DoTs to use on enemies. The effect list was expanded several times, both in size and splitting effects up into several categories in different lists.)
Shadowknights ask for more buff slots http://eqshadowknight.net/archive/index.php/t-1132.html
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Apparently, the designers of Everquest II decided to put a stop to this rampant buffing with several gameplay changes, one of which was to add a concentration mechanic.
- Characters now have a concentration bar with 5 segments. Most buffs, and some detrimental spells like Charm, take a certain number of your concentration segments. If you don’t have enough, you can’t use the ability. Most classes had too many buffs to cast them all at once, and had to pick and choose situationally.
- Most buffs were group-wide from the beginning. A few key buffs target just the caster, a single individual in your group or raid, or the entire raid.
- Group buffs are ranged - after casting, the recipient needs to stay within a distance of the caster (and be in the same zone), or the buff effect will end. Moving back to the caster will resume the effect.
- If the caster dies, changes zones, or logs out, the buffs ends for everyone and would need to be recast.
No more selling buffs, then? The cumulative result of these design changes is that buffing decisions in EQ II are much more strategic than in the previous Everquest - specific classes are valued for their buffs and must be included in the raid, and group configuration within the raid is crucial to ensure the right buffs get to the right raid members.
DnD
Prior to DnD 3.5, spellcasters were free to cast whatever spells they had available until they ran out of spell slots. This enabled a spellcaster to single-handedly set up fairly powerful combos, like turning the party rogue into a flying, enlarged, hasted storm giant.
Apparently this was judged to be too powerful, and in DnD 3.5 a concentration mechanic was added. Any spell with a duration of “Concentration” requires the caster to spend their action each turn in order to maintain the spell, preventing them from casting any other spells.
In DnD 5th Edition, more spells were altered to requre concentration, but the caster no longer needs to spend any of their actions to maintain the concentration, allowing them to cast other non-concentration spells.