Rise of Merlin is an exciting online slot game developed by Playn Go that features an intriguing twist to the familiar Book-series of games. Players are transported to a mystical world of dragons and magicians, where they spin the 5x3 reels for a chance to win big on 10 paylines. The game offers a free spins feature with expanding symbols that can be retriggered up to 9 times, promising the potential for huge winnings. Here are some of the main features and mechanics of Rise of Merlin:

Symbols on the reels:
- The crystal ball wild
- Merlin, the highest value symbol
- Red and blue dragons
- An owl
- Regular card suit symbols

Bonus Features:
- Free spins feature that can be activated by landing at least 3 crystal ball scatters on the same spin
- 8 free spins awarded initially
- A special symbol is chosen before the free spins start
- When the chosen symbol appears during free spins, it will expand to fill the entire reel
- The feature can be re-triggered by landing at least 3 new scatters during the free spins
- This awards 8 new spins with a new expanding symbol chosen
- Up to 9 retriggers possible, with 2 expanding symbols in play

Jackpot:
- The max win is 5,000 times the player's stake
- This can be achieved by filling the screen with the highest paying symbol

Other Details:
- Bets can be placed up to £100 per spin
- Highly volatile with an RTP slightly lower than Book of Dead
- The game is available on all devices and platforms
- No traditional bonus game outside of the free spins feature

In conclusion, Rise of Merlin offers an engaging gaming experience, particularly for fans of the Book-series. The addition of retriggerable free spins with expanding symbols keeps the gameplay fresh and exciting, and the chance to win up to 5,000x the player's stake is a great incentive.