Aegis of the Hearth

Aegis of the Hearth is a ritual spell most magi use to protect their homes and laboratories from other wizards' magic. Typically it is used to protect an entire Covenant, but there is nothing to prevent a magus from casting it to protect his sanctum or some other locale.

Protection

Aegis of the Hearth works (in Fifth Edition) a lot like Magic Resistance. It protects against spells and other supernatural powers, unless the caster or magic-worker participated in the casting of the Aegis (or has a special token given by the casters of the Aegis).

It also prevents creatures who have a Might Score below a certain limit from entering the protected area. Creatures or persons who lack a Might Score can enter the area normally. Note, however, that the aegis "blocks" spells from outside that fail to penetrate it, so that it may keep a character that is under such a spell from entering.

An often forgotten point is that the Aegis also affects magic items, and can block active enchanted devices or prevent items inside from activating their powers.

Aegis of the Hearth and Penetration

Technically, all spells in Ars Magica need to penetrate Magic Resistance in order to affect anything. Therefore, in order to keep a troll (for example) from entering an Aegis of the Hearth, the Aegis would have to penetrate its Magic Resistance.

In practice, many players waive the need for Penetration so they don't have to keep track of the Penetration Total of the Aegis for a whole game year, or because they feel the spell works better this way. This should be considered by the troupe.

Aegis of the Hearth and Hermetic Theory

Aegis of the Hearth is a powerful spell, and from a meta-game point of view if it were easy to modify there is a good chance it could be adapted into something that makes magi much too powerful. Therefore, as noted in the spell description, the spell can only be modified in certain limited ways. The in-game explanation for this is that Aegis of the Hearth is based on an ancient, pre-Hermetic ritual and is not fully explained by Hermetic Magic Theory.

Further Discussion

This thread on the official forum gives an in-depth discussion of how Aegis of the Hearth works, along with alternative interpretations and house rules.

References

Spell description, ArM5 page 161