Complex sentence: In 1810-1811 he edited for his brother John a quarterly magazine, the "Reflector", for which he wrote "The Feast of the Poets," a satire which gave offence to many contemporary poets, particularly William Gifford of the "Quarterly".
Split into simpler sentences: In 1810-1811 he edited a quarterly magazine, the "Reflector", for his brother John. He wrote "The Feast of the Poets" for this, a satire, which offended many contemporary poets, particularly William Gifford of the "Quarterly".