One of my favorite creatures from Fantastic Beasts & Where to Find Them is the thunderbird. This enormous bird is native to the deserts of Arizona and it creates storms by flying. Ilvermorny School of Witchcraft and Wizardry also uses the thunderbird as the symbol of one of its houses. Since it plays a prominent role in Fantastic Beasts & Where to Find Them I don’t want to spoil much about it so I will stick to the design aspect. I tried to play up the storm creation aspect while I was making the thunderbird, so I gave it ability to alter the weather at will, and I gave it immunity to thunder and lightning damage.
Huge beast, chaotic good
Armor Class 19 (natural armor)
Hit Points 215 (18d12+100)
Speed 15 ft., flying 100 ft.
Str 24 Dex 26 Con 30 Int 10 Wis 12 Cha 8
Damage Resistance bludgeoning, piercing, and slashing damage from non-magical weapons.
Damage Immunities lightning, thunder
Condition Immunities exhausted
Skills Acrobatics +12, Athletics 11, Intimidation +6, Perception +8
Senses passive Perception 13
Languages Understands Common but cannot speak.
Challenge 17 (18,000 XP)
Keen Sight. The thunderbird has advantage on Wisdom (Perception) checks made that rely on sight.
Magic Resistance. The thunderbird has advantage on saving throws against spells and other magical effects.
Storm Caller. The thunderbird can cast Control Weather without using components any number of times per day with the following modification. The thunderbird can only use the this ability to create storms, and rather than taking effect after 1d4x10 minutes the weather is altered immediately. It can also alter the weather by 2 levels rather than 1.
Multiattack. The thunderbird makes three attacks: one with its beak and two with its talons.
Beak. Melee Weapon Attack. +11 to hit, reach 5 ft., one target. Hit: target takes 16 (2d8+8) piercing damage.
Talon. Melee Weapon Attack. +12 to hit, reach 5 ft., one target. Hit: Target takes 13 (2d6+8) slashing damage.
Wing Attack (Costs 2 Actions). The thunderbird beats its many wings. Each creature within 10 feet of the thunderbird must succeed on a DC 19 Dexterity saving throw or take 12 (2d6 +7) bludgeoning damage and be knocked prone. After beating its wings this way, the thunderbird can fly up to half its flying speed.