It is HS&P's third new launch in less than a year, following the opening of dedicated consultancy Loyalty Practice and data specialist Data.HS&P.
HS&P's founder and CEO Justin Hicklin will extend his duties to head up the new company.
Legends will focus on creating "valuable consumer conversations for brands", based on HS&P's word of mouth experience with brands including Diageo, Honda and Which?.
A team of analysts and planners will aim to immerse themselves in consumers' worlds, experiences and conversations in order to create engaging brand stories that people will be keen to spread.
Legends will measure the impact of word of mouth on individuals, brands and the business.
Hicklin said: "Today's WOM has grown up -- creating a buzz is one thing, but you must know who you need to talk to in order to make that buzz and ultimately generate engagement with the right people, and drive sales cost-effectively.
"This was a natural addition to HS&P's rapidly developing group of companies. Helping us to deliver consumer insight through another channel, Legends heralds the start of just one more way we work with brands to build measurably better relationships."