From The Heroes of Might and Magic III wiki

An Adventure map structure a hero can visit and receive Wyverns.

The hero must defeat Dragon Fly guardians. More guardians means a better reward.

Probability %	Guardians	Reward
30	30 Dragon Flies	4 Wyverns
30	45 Dragon Flies	6 Wyverns
30	60 Dragon Flies	8 Wyverns
10	90 Dragon Flies	12 Wyverns

As with all Creature Banks, the troops are surrounded rather than facing the guardians across the normal battlefied, and no War Machines are used.

If only some of the guardians are killed they do not reset.