It's actually fairly simple.
First of all the basic norse favor gathering is determined by a very simple formula. Each unit is "worth" a certain amount of favor. You take how much it's worth times the amount of damage you did to it over it's max HP, and you get the amount of favor.
Let's do a very hypothetical example (no, I do not know for a fact that ulfs are worth 2.0 favor). Say you do 20 hitpoints of damage to an ulf, with any norse unit. You would do this:
2.0 * 20/90 = .444 favor. Now, I doubt ulfs are worth that much favor, but it's a hypothetical example.
Also, you must note that hersir gather 2x more favor than any other norse unit, so they would get .888 favor from that ulf.
The hersir has only a few myth it can summon. It can only summon a myth unit from the age it was built (the myth has to be norse, naturally), so if you have 4 archaic hersir and one classical hersir, your chances of summoning are less but not drastically.
Please keep in mind the fact that the only way you can get favor is by killing and/or damaging things, this applies to all norse units, if your hersir die without inflicting any damage, they didn't collect any favor for the bank or for the pool.
Also, when a hersir is built it is auto assigned a myth. The myth is random from the age the hersir is built. You don't actually have a one in 3 chance of summoning a valk. Statistically, yes, but 4 of your 5 hersir can be assigned einherjar, DRASTICALLY reducing the chances of summoning a valk.
Hersir get favor in 2 ways - to your resource "favor", and to a favor pool which is hidden from the eye. All hersir are connected to this pool. If the pool has 25 favor and a hersir with the battle boar assigned to him attacks something, you will summon a battle boar. Favor gathering for this pool is slower on buildings.
This pool favors myth that are cheaper, because it will be hard to allow that pool to build up to 26 favor for a fire giant, because the hersir assigned to an einherjar will summon at 14 favor. And, just as a note, the pool can only be filled by the hersir, not TA's, RC, or any other unit.
Even though a hersir was built in the archaic age, it does not mean that he does not add to the pool. ALL hersir add to the pool, and any hersir can withdraw from it to summon. An archaic hersir can earn 100 favor for the pool, but have it all used up by another hersir to summon a troll or something.
Myth summoned are dependent on population. If you're popped with 70 favor in the pool, and pop suddenly opens up the first hersir to attack will summon, no matter what. If you're pool is too low, he will not summon.
This is why massed hersir armies in the late ages very rarely work, because you're constantly filling your slots with new units, so there's no pop for myth. Eventually the pool will hit a maximum and all those hersir would have been wasted.
Favor added to the pool is determined in the same way as regular favor added to your bank. In fact, favor added to the pool comes in FASTER than in the regular pool, but not by much. However, the favor a hersir naturally gathers just be being alive does NOT affect the favor pool (they don't want hersir standing around, then whacking one guy and summoning a fire giant).
The myth units are not free. It's like taking away the gold cost of an einherjar and just paying for the favor. You have to EARN the favor. The reason you don't see myth popping up like crazy is because 5 or 6 units of a 30 unit army (the 5-6 hersir you might have in your army) are not earning a hell of a lot of favor. Your hersir will develop maybe 30% of your favor MAXIMUM in the first 10 minutes of the rush (unless you really flooded them), and that's 50 favor tops. 3 einherjar, 2 valks, 1 einherjar and a valk. You'll notice that this is probably what you end up summoning early on, or even less because your hersir aren't earning all that much favor, it's the other units that are filling the bank.
So, really, the game does not decide anything, the circumstances just have to be right. There has to be enough favor in the pool, enough pop, and a hersir assigned to a myth unit whose cost is equal to or less than the amount of favor in the pool to summon.
[This message has been edited by Einstein_006 (edited 10-18-2003 @ 08:17 PM).]