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Here, a serious problem arises. In theory, it sounds good, but in practice, there will be issues with the animation. But it crawls, so it crawls.
My opinion. The snake should be the fastest and, consequently, the most proactive. Since the Basilisk crawls (including through walls), its initiative should be somewhere in the middle (among all swamp creatures). Plus, it would make sense to add a special ability, such as burrowing into the ground and emerging in a certain area of the battlefield, and then attacking a unit on the target square. The Basilisk appears from the ground with a delay of 1-2 turns; if it appears on the next turn, it will create an imbalance.
2nd option. By the way, it would be more convenient to make it so that it doesn't crawl through walls, but burrows into the ground and appears within its own movement range (and not anywhere on the map), followed by an attack. Another example of a special ability: the Basilisk swallows N units (the number of swallowed units will depend on the level of the creatures and the number of Basilisks) (in addition to the attack) and digests them; the health of the swallowed creatures will decrease according to the formula: number of Basilisks * a certain fixed and yet-to-be-chosen number per turn. When the Basilisks are killed, the remaining undigested creatures are released and added to the squad. The special ability works once per battle. However, I think I'm getting carried away.