I don't think you need the "then" after the semi-colon

I believe it would just be "When this card is Link Summoned: You can banish all DARK monsters this card points to; this card gains 600 ATK for each monster banished by this effect." or you could write it as just "When this card is Link Summoned: You can banish all DARK monsters this card points to, and if you do, this card gains 600 ATK for each monster banished by this effect." Not an expert on PSCT so don't take me at my word. Other than that thought it's a nice card.