when it comes to; as for; in the case of
Used to single out an extreme or notable example, often with a sense of 'even' or 'especially'. It can carry a negative or surprising nuance, implying that the mentioned case is particularly remarkable or problematic.
See also: 至る
He is usually gentle, but when it came to that, he exploded in anger.
この店の料理はどれも美味しいが、に至っては絶品だ。
All the dishes at this restaurant are delicious, but this one in particular is exquisite.
Both introduce a topic, but に至っては emphasizes an extreme or surprising example, while となると is more neutral and conditional ('if it comes to').
Derived from the verb 至る (いたる, 'to reach') in the te-form with the topic particle は, literally 'having reached (the point of)'. It grammaticalized into an expression meaning 'when it comes to (an extreme case)'.