This is the 8426th most frequent Japanese word.
惨たん
"惨たん" (santan) means "deep sorrow" or "grief."
惨たん is used here as an adjective describing a tragic and dreadful situation.
その惨たんな状況は誰も想像できませんでした。
Nobody could have imagined such a tragic situation.
惨たんたる here is used to express profound sorrow or despair caused by a particular circumstance.
人々の無関心に惨たんたる思いを感じた。
I felt deep despair at the people's indifference.
惨たんとしている emphasizes the bleak and lamentable nature of the fact.