Sigil Captain's ability has an "intervening 'if' clause." That means (1) the ability won't trigger at all unless the creature that enters under your control is 1/1, and (2) the ability will do nothing unless that creature is still 1/1 at the time the ability resolves.
2009-05-01
wotc
Sigil Captain's ability checks a creature's initial power and toughness upon being put on the battlefield, so it will take into account counters that it enters with and static abilities that may give it a continuous power boost once it's on the battlefield (such as the one on Glorious Anthem). After the creature is already on the battlefield, changing its power and toughness to 1/1 with a spell, activated ability, or triggered ability won't allow this ability to trigger; it's too late by then.
2009-05-01
wotc
If, for some reason, Sigil Captain enters as a 1/1 creature (due to a pair of Engineered Plagues, for example), it will cause its own ability to trigger.
2009-05-01
wotc
For that reason, two Sigil Captains don't work well together. When a 1/1 creature enters under your control, the abilities of both Sigil Captains will trigger. When the first resolves, two +1/+1 counters are put on the 1/1 creature. When the second resolves, it will do nothing because the creature that entered is now 3/3.
Format Legality
Standard
Not Legal
Future
Not Legal
Historic
Not Legal
Timeless
Not Legal
Gladiator
Not Legal
Pioneer
Not Legal
Modern
Legal
Legacy
Legal
Pauper
Not Legal
Vintage
Legal
Penny
Legal
Commander
Legal
Oathbreaker
Legal
Standardbrawl
Not Legal
Brawl
Not Legal
Alchemy
Not Legal
Paupercommander
Not Legal
Duel
Legal
Oldschool
Not Legal
Premodern
Not Legal
Predh
Legal
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