Who is "Bellona's bridegroom?" in Act I of Macbeth?
First, it is necessary to define who "Bellona" is. She was the Roman goddess of war, and her husband, according to Roman mythology, was Mars, the god of war. So by referring to the man (which, as will be seen below, is ambiguous) who faced off against Cawdor as "Bellona's bridegroom," Ross means to compare him favorably to a god of war:
Assisted by that most disloyal traitor
The Thane of Cawdor, began a dismal conflict,
Till that Bellona's bridegroom, lapp'd in proof,
Confronted him with self-comparisons,
Point against point rebellious, arm ’gainst arm,
Curbing his lavish spirit; and, to conclude,
The victory fell on us.
The problem is that it is not clear who Ross is referring to, since he is describing events that went on at Fife, which is separate from the fight in which Macbeth kills the rebel Macdonwald. Macbeth, as we learn later, does not even know of the defeat and death of Cawdor until Ross tells him about it (thus confirming the witches' prophecy.) It is possible that Ross is actually not referring to an individual, but speaking metaphorically, saying that Cawdor didn't possess the military prowess to win the battle on the day. Still another intriguing possibility is that, since the battle occurred at Fife, Cawdor may have been defeated by the Thane of Fife, none other than Macduff, who would have been much displeased that the title of Thane of Cawdor went to Macbeth.
Earlier in the scene, the Captain described for King Duncan the bravery and courage of Macbeth (and Banquo) on the battlefield: how, first, Macbeth fought against the traitor Macdonwald, brutally killing and beheading him, and, then, how Macbeth turned to face the Norwegian king's forces next. After this, Duncan sends the Captain for medical help.
When the Captain leaves, the Thanes of Ross and Angus enter, having come from Fife, and Ross describes the conflict between "Bellona's bridegroom" and the Thane of Cawdor, another traitor, fighting on the side of Norway. We don't know precisely where the Captain came from, where he saw Macbeth fighting, but it doesn't seem to be the same location from which Ross has come. Ross says that, in Fife, "the Norweyan banners flout the sky," and the Captain provided no such description of the location from which he came (1.2.49).
Further, Ross never uses Macbeth's name specifically, and so it seems less and less likely that the reference to Mars, the god of war, (and Bellona's husband) is being applied to Macbeth. Because Ross has already named Fife, it seems more likely that this allusion refers to Macduff, the Thane of Fife, who we later discover to be a formidable warrior himself when he bests and beheads Macbeth in combat. We cannot, however, be totally sure.