The problem is with the sequence of tenses in the two clauses. In one clause you are using the past perfect and in the other the present perfect. You can see the problem if you switch out of the perfect in the second clause.
"The enmity ... had reached a level where reconciliation becomes impossible" mixes up a statement about an event that preceded an event in the past with a statement that pertains only to the present. It makes no sense.
Any of the following are fine.
"The enmity had reached a level where reconciliation had become ..." put both ideas into a past that precedes some other past event, perhaps a divorce. Both clauses in the past perfect.
"The enmity ... reached a level where reconciliation became ..." indicates past events. Both clauses in the simple past.
"The enmity ... has reached a level where reconciliation has become ..." indicates a continuation into the present of a situation that started in the past.