Arcane Foci (3.5e Variant Rule)
From D&D Wiki
This variant rule adds a hindrance to arcane spellcasting. All spells receive a 20% failure chance, and attack spells take -1 on attack and damage rolls. Divinations have a 20% chance to mislead you, and conjurations that involve living beings have a 20% chance to summon either the wrong creature or summon a creature that attacks the caster and his comrades. All these restrictions can be bypassed through the use of an arcane focus. These need not be carried by the caster; they can be in a safe chest at home and still work. Arcane focuses can be anything from a modestly carved orc tooth to a life-sized elegantly carved solid gold and platinum statue of the greatest paladin of the multiverse with perfectly spherical diamonds for eyes. A bards arcane focus, however, is always an instrument. Any new spell a wizard, bard, or sorcerer learns must be attuned to the focus to receive these benefits; before this is done, that spell receives all the penalties described above. Attuning a spell to a focus requires you to grasp your focus tightly and think of the spell as if you were about to cast it. Attunement requires 1 minute/spell level, and an arcane spellcasters initial attunement to their focus requires 2 hours plus an hour per point of Intelligence bonus.