Is buying a Legendary dragon enough to get stronger?

No — and it's a common trap. Buying a Legendary dragon (around €17.99 in the store) gives you the dragon, but not its final power.

What actually makes a dragon strong:

• Its Star Rank (stats plus unlocked Habit slots)
• Its Habits leveled 1→5, especially the ⭐ set Habit that reinforces its Order
• Its affinity lined up with your army (a Legendary in the wrong army loses the triangle edge)

A freshly bought Legendary at 1★ with no Habits can be less useful than a humbler but well-developed dragon. A telling example from Adel's roster: Jagadrix, with the lowest 🔥 Power, stays excellent thanks to its stars and active effects.

The takeaway: owning ≠ developing. Budget the Relics/Cores to grow it too, or the purchase just sits there.