Deoxys (5e Creature)
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Medium aberration (shapechanger), chaotic neutral
Armor Class 16 (natural armor) in Normal form, 14 in Attack form, 20 (natural armor) in Defense form, 16 in Speed form
Skills Arcana +10, Perception +8
Pressure. All creatures of Deoxys's choice within 60 feet of it have disadvantage on Constitution saving throws.
Shapechanger. Deoxys can use a bonus action on its turn to shift into its attack, defense, or speed form; or back into its normal form. Its statistics, other than its AC, are the same in each form, except for the following exceptions:
Any equipment it is wearing or carrying isn't transformed.
Mold Breaker (Attack form only). Deoxys's attacks ignore damage resistance.
Magic Resistance (Defense form only). Deoxys has advantage on saving throws against spells and other magical effects.
Improved Initiative (Speed Form only). Deoxys has advantage on initiative rolls. If the campaign uses static initiative, it instead gains a +5 bonus to its initiative score.
Innate Spellcasting (Attack form only). Deoxys's innate spellcasting ability is Intelligence (spell save DC 20). It can innately cast the following spells, requiring no components:
1/day: psychic scream
Multiattack. Deoxys makes two attack, either with its Psycho Boost or with its Zen Headbutt. If it is in its speed form, it can instead make three attacks.
Psycho Boost. Ranged Spell Attack: +10 to hit (+12 in attack form), range 20/60 ft., one target. Hit: 31 (6d8 + 4) psychic damage (42 (8d8 + 6) in Attack form).
Zen Headbutt. Melee Weapon Attack: +10 to hit (+12 in attack form), reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage (24 (4d8 + 6) in Attack form), plus 18 (4d8) psychic damage.
Deoxys can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Deoxys regains spent legendary actions at the start of its turn.
Shift. Deoxys shifts into its Attack, Defense, Speed, or Normal form.
Detect. Deoxys makes a Wisdom (Perception) check.
Attack (Attack form only, Costs 2 Actions). Deoxys makes a Psycho Boost or Zen Headbutt attack.
Cosmic Power (Defense form only, Costs 2 Actions). Deoxys gains resistance to two damage types of its choice until the start of its next turn.
Move (Speed form only, Costs 2 Actions). Deoxys moves up to its speed without provoking opportunity attacks.