You can open the Golden Pinata with 
Golden Pinata Truncheon.
You can get the Golden Pinata Truncheon from event's progress bars.
Golden Pinata - Rewards per Location
Rewards :
- Splintered Gem x5
Essence of Vigor x2000
Anniversary Arena Chip x5
Phestos' Pinata Truncheon x8- Mighty Spirit Guard x10
- 250
- One of the following Jackpot rewards (one time rewards):
- Pauldrons of the Ferocious Beast + 500
- 2000 + 1000 + 2000
- Gold Squire + 500
- Lesser Runes x5
- Celestial Dragon + 500
- Flawless Gem x3 + Sacred Gem
- Crystal of Truth x777
Rewards :
- Simple Gem x5
- Essence of Vigor x3000
- Anniversary Arena Chip x5
- Phestos' Pinata Truncheon x8
- 250
- One of the following Jackpot rewards (one time rewards):
- Pauldrons of the Ferocious Beast + 500
- 2000 + 1000 + 2000
- Gold Squire + 500
- Lesser Runes x5
- Celestial Dragon + 500
- Flawless Gem x3 + Sacred Gem
- Crystal of Truth x777
Rewards :
- Normal Gem x5
- Essence of Vigor x2000
- Anniversary Arena Chip x5
- Phestos' Pinata Truncheon x8
- Mighty Spirit Guard x10
- 250
- One of the following Jackpot rewards (one time rewards):
- Pauldrons of the Ferocious Beast + 500
- 2000 + 1000 + 2000
- Gold Squire + 500
- Lesser Runes x5
- Celestial Dragon + 500
- Flawless Gem x3 + Sacred Gem
- Crystal of Truth x777
Rewards :
- Polished Gem x5
- Essence of Vigor x4000
- Anniversary Arena Chip x5
- Phestos' Pinata Truncheon x12
- 250
- One of the following Jackpot rewards (one time rewards):
- Pauldrons of the Ferocious Beast + 500
- 2000 + 1000 + 2000
- Gold Squire + 500
- Lesser Runes x5
- Celestial Dragon + 500
- Flawless Gem x3 + Sacred Gem
- Crystal of Truth x777
Rewards :
- Polished Gem x10
- Essence of Vigor x2000
- Anniversary Arena Chip x5
- Phestos' Pinata Truncheon x16
- Mighty Spirit Guard x10
- 250
- One of the following Jackpot rewards (one time rewards):
- Pauldrons of the Ferocious Beast + 500
- 2000 + 1000 + 2000
- Gold Squire + 500
- Lesser Runes x5
- Celestial Dragon + 500
- Flawless Gem x3 + Sacred Gem
- Crystal of Truth x777
Rewards :
- Polished Gem x10
- Essence of Vigor x4000
- Anniversary Arena Chip x5
- Phestos' Pinata Truncheon x18
- 250
- One of the following Jackpot rewards (one time rewards):
- Pauldrons of the Ferocious Beast + 500
- 2000 + 1000 + 2000
- Gold Squire + 500
- Lesser Runes x5
- Celestial Dragon + 500
- Flawless Gem x3 + Sacred Gem
- Crystal of Truth x777
Rewards :
- Polished Gem x10
- Essence of Vigor x5000
- Anniversary Arena Chip x5
- Phestos' Pinata Truncheon x20
- 250
- One of the following Jackpot rewards (one time rewards):
- Pauldrons of the Ferocious Beast + 500
- 2000 + 1000 + 2000
- Gold Squire + 500
- Lesser Runes x5
- Celestial Dragon + 500
- Flawless Gem x3 + Sacred Gem
- Crystal of Truth x777