Thunderbird
About
The thunder is its wings and the lightning is its eyes, and the storm is not something it brings with it - the storm is what it looks like from underneath. It carries whales out of the sea in its claws when it wants to. It is not a monster in the sense of a thing to be gone and dealt with; it is closer to a season, and the people who live under its country plan around it, and have opinions about outsiders who talk about hunting it.
Stats
Save Rolls
Traits
Special Attack
It has two attacks each round and may spend either or both on Lightning, so a round may be two Talons, two Lightnings, or one of each. Lightning: 100 ft, save Dex Quick, fail save roll 3d8+6 lightning. Talons: slashing, melee 20 ft, +6 to hit, 3d8+6, and it takes things away rather than killing them where they stand.
Special Defense
The Storm Is It: within its weather, visibility is poor, footing is bad, flight is impossible for anything else, and ranged attacks are at Disadvantage. This is not a spell and cannot be dispelled or waited out. Immune to lightning and thunder. Not an Enemy: it has no quarrel with anyone and does not initiate, and a party that shelters and lets it pass is not touched. Reduced to 0 HP it falls, the weather goes with it, and the DN should decide what the country does about that, because the people who live there will have views.