source: [食戟のソーマの薙切えりなが女王として君臨するGIF画像｜無料GIF画像検索 GIFMAGAZINE 630650]

hold royal office; rule as king or queen:

〈王･女王が〉君臨する, 在位する

e.g. Queen Elizabeth reigns over the UK.

be the best or most important in a particular area or domain:

«…で» 優位を築く «over»

e.g. in America, baseball reigns supreme.

(of a quality or condition) prevail; predominate:

｟文｠ 〈状態･考えなどが〉 «…に» はびこる, 行き渡る

e.g. confusion reigned.

the period during which a sovereign rules:

(王･女王の)治世, 統治期間; 君臨, 支配

e.g. the original chapel was built in the reign of Charles I.

the period during which someone or something is predominant or preeminent:

(人･考えなどの)優位, 優勢, 支配(の時代)

e.g. these historic seconds inaugurated the reign of negative political advertising.

Middle English: from Old French reignier ‘to reign’, reigne ‘kingdom’, from Latin regnum, related to rex, reg- ‘king’.

The correct idiomatic phrase is a free rein, not a free reign; see usage at rein.