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Endoperez said:
These effects are called Indirect magic in the manual.
Fire - Attack +1, Leadership +5
Air - Precision +1
Water - Water breathing for the mage, can take one additional unit under water per Water level above 1 (e.g. W3 mage can have two Knights go underwater with him).
Earth - Protection +1 (natural protection, doesn't stack fully with armor)
Astral - extra +5 magic leadership (total +10)
Death - 20 Undead leadership, but NO magic leadership at all! Also the Fear bonus.
Nature - Supply +5
Blood - Undead leadership +5
Unholy also gives Undead leadership +10.

Some paths get more powerful boni than others, but everything has something. All paths except Death also get +5 Magic leadership.

EDIT: Oops, Wick got before me. Well, at least I get to correct him, as only Astral gives +10 in Magic leadership.

And the listed bonuses are per magic level, and stack with each other.

For Water it's not eactly that according to my experience : a Niefel Jarl (W3) *cannot* have 2 Niefels with him underwater, only 1... So I suppose the full rule is that each level of Water magic allows for 2or 3 unit-size to accompany the leaders. If you can get 2 knights with W3 it's 3...