As long as you control The Goblin Mastermind or it's your
commanderA designated card for a Commander deck, governed by the command-zone, color-identity, and Commander rules. (CR 903), permanents you control are Kindred Goblins in addition to their other types. The same is true for spells you control and cards that you own that aren't on the battlefield.
Whenever The Goblin Mastermind deals combat damage to a player, you
createTo create one or more tokens with certain characteristics, put the specified number of tokens with the specified characteristics onto the battlefield.
If a replacement effect applies to a token being created, that effect applies before considering any continuous effects that will modify the characteristics of that token. If a replacement effect applies to a token entering the battlefield, that effect applies after considering any continuous effects that will modify the characteristics of that token.
Previously, an effect that created tokens instructed a player to “put [those tokens] onto the battlefield.” Cards that were printed with that text have received errata in the Oracle card reference so they now “create” those tokens. (CR 701.7) a token that's a copy of one of Skirk Prospector, Goblin Lackey, Goblin Piledriver, Goblin Warchief, Boggart Harbinger, or Goblin Ringleader, chosen at random.