Douglas Harper's Etymology Dictionary
"formal declaration of sympathy," 1670s, plural of condolence. Reason for the plural is unclear; earliest references are to expressions from groups of persons; perhaps the habit stuck.
Wiktionary
interj. (non-gloss definition: Expressing perfunctory or jocular condolence wishes.) n. 1 (plural of condolence English) 2 (context plural only English) comfort, support, or sympathy offered to the family and friends of somebody who has died
Wikipedia
Condolences (from Latin con (with) + dolere (sorrow)) are an expression of sympathy to someone who is experiencing pain arising from death, deep mental anguish, or misfortune.
The use of the word "condolences" is more common than "condolence". The reason for this situation is unclear.
When individuals condole, or offer their condolences to a particular situation or person, they are offering active, conscious support of that person or activity. This is often expressed by saying, "Sorry." Often, the English language expression "My condolences" will be in a context, such as death of a friend's loved one, in which the one offering condolences is communicating a feeling of pity towards the unfortunate party. Expressions of condolence may also be used as sarcasm if one considers the complaining person to be exaggerating their suffering, especially when they appear to consider as tragic something that is relatively insignificant.
Condolence is not always expressed in sorrow or grievance, it can also be used to acknowledge a fellow feeling or even a common opinion.
There are various ways of expressing condolences to the victims. Examples include donating money to the charity nominated by the person who has just died, writing in a condolences book or supporting the friends and family of the loved one by making meals and looking after them in various ways in times of need.
Usage examples of "condolences".
His servant situated his folding chair and literally lowered his master down onto it, while those around him murmured their condolences and he nodded, nodded, nodded his thanks, face almost as white as hers had been.
Mother hurried back to Spring Lake to express her condolences and found the family to be in a state of profound grief.