Not to be confused with 事（こと） ‘thing’, 殊（こと） is a 副詞（ふくし） (adverb) with the same reading that often appears in hiragana, specifically to highlight that something is to a ‘great extent’. Due to being used adverbially, 殊（こと） will be followed by に. The kanji 殊（こと） itself has the meaning of ‘particularly’.
This expression is regularly seen after verbs, い or
な-Adjectives. When 殊（こと） has the meaning of ‘particularly’, ‘especially’, or ‘to my (A)’, it is almost always describing some kind of feeling or emotion.
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ビックリすることに、久（ひさ）しぶりに小学生（しょうがくせい）時代（じだい）の友達（ともだち）から電話（でんわ）がかかってきた。
I am especially surprised that I got a phone call from a friend from elementary school after such a long time.
珍（めずら）しいことに、彼（かれ）がいいレストランに連（つ）れていってくれた。
It is particularly rare to be taken to a nice restaurant by him.
残念（ざんねん）なことに、その商品（しょうひん）はもう販売（はんばい）中止（ちゅうし）になりました。
Unfortunately, that product is no longer being sold.
In most situations, ‘(A)
ことに (B)’ can be translated quite literally as ‘to a particularly (A) extent, (B)’.
A very common expression
幸（さいわ）いなことに uses this particular grammar structure as a set phrase, and is often translated simply as ‘fortunately’, or ‘luckily’.
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幸（さいわ）いなことに、近所（きんじょ）の人（ひと）が私（わたし）の猫（ねこ）を見（み）つけて家（いえ）まで届（とど）けてくれた。
Fortunately, my neighbor found our cat for us, and brought her home.
Although it is quite uncommon, こと may also be seen as 異（こと）, a kanji meaning ‘irregular’.