taru-adjective, adverb taking the 'to' particle
Literary bound form used with たる or と to describe a situation or sight that is shockingly bad, miserable, or horrifying. Not used in everyday conversation; typical in written narratives.
戦場の惨たる光景が目に浮かぶ。
The appalling sight of the battlefield comes to mind.
惨として語るに忍びない事件だった。
It was an incident so dreadful that one cannot bear to speak of it.
悲惨 is the common word for 'miserable' or 'tragic' in modern Japanese. 惨 is a bound literary form used in fixed expressions like 惨たる or 惨として.
惨め is a standard adjective for personal misery or wretchedness. 惨 is more objective and literary, describing external scenes or events.
The kanji 惨 carries meanings of 'miserable' and 'cruel'. The reading さん is the on'yomi, used in literary compounds and bound forms. The exact historical derivation of the bound adjectival use is uncertain, but it follows classical Chinese-influenced patterns.