how Tahm Kench works?

just tried a game with Tahm Kench

Quite shocked that instead to strike an enemy before swallowing him, the two fought ... with the result that Tahm Kench died ...

how does he work?

I'm not entirely sure what you mean. If the two fought do you mean like Single Combat ? If you do then that would be a bug, but it would be one that is in your favour since Tahm Kench is only meant to take the damage of the enemy minion. Then if he survives said damage he captures the unit that struck him.

1.) You have a Tahm Kench on board at the start of a round; he generates 1x fleeting An Acquired Taste

2.) You play that An Acquired Taste on an enemy unit

  2a.) First half of Taste: that unit strikes your Tahm Kench

  2b.) Second half of Taste: if TK survives, that unit is detained

3.) If TK levels or L2 TK attacks: the detained unit is permanently obliterated. Otherwise, it will return instantly if TK is killed, obliterated, or recalled.

Smart opponents will take advantage of 2a and use that opportunity to either damage TK or buff their unit's attack such that TK doesn't survive for 2b. It is good to try to keep some buffs in hand so you can boost your TK's health and/or heal him up. This is part of what makes TK + Soraka such a powerful pair.

Another good pairing is to hit Tahm Kench with Unyielding Spirit. It turns him into a really annoying opponent