Wiktionary
vb. 1 (context intransitive English) To burn brightly again. 2 (context intransitive English) To become more intense suddenly. 3 (context intransitive sometimes with “out” instead of “up” English) To burst out suddenly, as in anger.
WordNet
v. ignite quickly and suddenly, especially after having died down; "the fire flared up and died down once again"
erupt or intensify suddenly; "Unrest erupted in the country"; "Tempers flared at the meeting"; "The crowd irrupted into a burst of patriotism" [syn: erupt, irrupt, flare, break open, burst out]
Usage examples of "flare up".
Every clan of the eastern tribes had a feud with somebody waiting to flare up again, and everyone was related to everyone else by descent or marriage or blood brotherhood, so a single killing could sprawl out into an uncontrollable free-for-all of ambushes and lethal brawls like a sweater unraveling from a single tug.
She was very stooped and round-shouldered from osteoporosis, and her arthritis seemed to flare up more often.
Something happens to cause wild magic to flare up and the investigating wizards are all killed.