The secret effect:
Summoning 6 cat minions while Headmaster Kel’Thuzad is in play will add a copy of Mr. Bigglesworth to your hand. (0 Mana, 1/1 Beast).
The list of ‘cat’ minions can be found in the interwebs.
As for the bug:
- Internally, Headmaster Kel’Thuzad uses the same counter for both his Spellburst having been triggered and the amount of cats summoned to get Mr. Bigglesworth, which causes buggy interactions when both of Headmaster Kel’Thuzad’s triggers are used simultaneously. If his Spellburst triggered first, you will only need to play four cats to get Mr. Bigglesworth. If you played a cat minion first, then playing even a non-spell card will trigger his Spellburst. This bug with cats acting as Spellburst triggers was fixed with [Patch 18.2.0.58213]