Complex sentence: There were 13 participants; they included Charles Stewart, 3rd Marquess of Londonderry (who brought out an umbrella) and Henry Beresford, 3rd Marquess of Waterford, who wore a reproduction suit of armour.
Split into simpler sentences: There were 13 participants; one was Charles Stewart, 3rd Marquess of Londonderry (who brought out an umbrella). Another was Henry Beresford, 3rd Marquess of Waterford, wearing a reproduction suit of armour.