They fire huge flaming rocks at Saradomin's units, which deal a large chunk of damage to them. The Siege beasts are capable of dealing up to 32,000 damage on their Saradominist counterparts, the colossi. Like all special units, they do not retaliate against players unless they are not currently under attack or are attacking any other target.
It would have appeared earlier on in the battle depending on the outcome of the second Zamorakian vote. However, the second vote resulted in favour of the Zamorakian champion instead. The eighth vote again gave the behemoth the possibility of appearing in battle, but instead resulted in favour of the Zamorakian butcher demon, again preventing its entrance to the battle. However, on the final week of the battle, it was finally released, and it could be found standing by the entrance to Zamorak's camp.
Killing it gives six times the player's current multiplier in divine tears. Merely participating in the kill is enough to yield the tear reward - it is not required to deal the most damage.