How to unlock Hydreigon
Getting Hydreigon means pulling the Unova banner and hitting its 5★ slot. The 5★ band takes 1.5% of all pulls and holds 103 partners, so one specific Hydreigon works out to about 0.0146% per draw. The 70-pull pity counter guarantees a 5★, but it does not target Hydreigon specifically, so a guarantee can still resolve to another 5★. A duplicate Hydreigon refunds 300 coins at the 5★ rate.
Hydreigon stats and rank
Hydreigon carries 111 HP, 119 speed, a ×1.39 attack multiplier, ×1.28 physical, ×1.39 special, 90 defense and 90 special defense, 600 in total. On the three-stat percentile this is 84.2 out of 100, which puts Hydreigon at rank 45 of 966 and in tier S. What separates it from its neighbours is shape, not size: its strongest slot is special multiplier (93rd percentile) and its weakest is defense (73rd percentile), so it spreads its value fairly evenly.
Best moves in Hydreigon's pool
At move level 5 the pool is led by Dragon Claw at 48.3 damage per 0.39s (158.5 effective DPS, ×1.28 multiplier), then Body Slam at 75.6 damage per 0.71s (135.8 effective DPS, ×1.28 multiplier), then Bite at 35.7 damage per 0.34s (135.4 effective DPS, ×1.28 multiplier). 10 of its 16 damaging moves scale with the physical multiplier and 6 with the special one, alongside 5 support moves. Its special multiplier is the higher of the two (×1.39 against ×1.28), so special moves convert better. The top option beats the second by 22.7 DPS, which decides slot one and leaves slot two as a judgement call.
Evolution cards for Hydreigon
Its starting move, Bite, evolves into Crunch once the move is at level 5 and you hold the Power card, which boosts attack for that run. Menu upgrades use a separate system and never stand in for Power. 16 of its 21 moves carry an evolution card, across 6 distinct cards (Vigor, Sprint, Power, Reflexes, Armor and Synthesis). The strongest of them is Dragon Claw at 158.5 effective DPS, so that is the one worth holding a card for first.
Hydreigon compared to similar partners
The two closest stat lines in the roster are Moltres (-1 HP, -3 speed, same attack multiplier, -20 total) and Zapdos (-1 HP, +1 speed, same attack multiplier, -20 total). Against Moltres, Hydreigon is 3 speed faster and the two carry an identical ×1.39 attack multiplier, and the rank gap is 30 places. Read those deltas as swap costs, because a gain on one line is usually paid for on another, so neither side is a strict upgrade and the move pools have to break the tie.