Karl Barth's Church Dogmatics is an unfinished summa theologica, and theological summas are almost always unfinished, and end unexpectedly and abruptly. So it was with Karl Barth as well. In the Christmas season of 1964, Karl Barth suffered a stroke that robbed him of his speech for half a day. Barth said he interpreted this event as a sign from God telling him that he had said enough, and it was time to stop writing.
In the autumn of 1965, almost a year after his stroke, Barth finally returned to his writing desk, but during this same time period, his "faithful assistant" Charlotte von Kirschbaum was hospitalized due to Alzheimer’s disease , and Barth realized he was unable to continue the Church Dogmatics without her (demonstrating that "lollo" was far more than a "faithful assistant").
Karl Barth's Final Words in the Church Dogmatics
In May of 1966, Barth turned 80 years old, and said that he had reached the upper limit of life, according to Moses (Ps 90:10), and afterwards described himself as the "late Barth". At that time, Barth returned to the Church Dogmatics volume four, part four, that he had started before his stroke, and published the unfinished fragment (CD IV/4) on Baptism in 1967. Barth's remaining lecture notes for the unwritten end of CD IV/4 have been published, but Barth never started the fifth and final volume of the Church Dogmatics. So the final written words in the official Church Dogmatics is the preface Barth wrote in the preface of CD IV/4, in which Barth explains his reasons for abandoning the Church Dogmatics. Here is a quotation of these final words:
When this was happily behind me, there began an indisposition which gradually developed into a regular illness in the course of which I had several periods in hospital and many operations, and caused much trouble and concern to the doctors and nurses and also to my own family. In spite of this, or perhaps because of it, I look back on this time with thankfulness, and I was able to read a great deal, though not to write. It ended in the late autumn of 1965, when, reasonably restored to health, I was able to return to my desk.
Mention may be made of one notable incident. Shortly before Christmas 1964 I had a slight stroke which for half a day robbed me of speech—perhaps a sign in view of the much too much that I have said in my lifetime. Then, possibly in unconscious protest against the undue disparagement of the third Evangelist by ruling New Testament scholars, and certainly to the edification of the deaconess who was caring for me, the name Zacharias (Lk. 122) clearly passed over my lips in description of my state. Quite soon afterwards I was able to say more about the situation. Nothing like this has happened to me since—not yet!
It so happened that my faithful assistant Charlotte von Kirschbaum, who had been indispensable from 1930 onwards, suffered an even more serious illness than mine (definitively from the end of 1965 and beginning of 1966), so that she was out of action in relation to the Church Dogmatics, in whose rise and progress she had played so great a part. Furthermore, I myself was now a little older. I celebrated my eightieth birthday in May 1966. . . . I mention these points here in order that it may be commonly understood why I could not think of carrying forward the Church Dogmatics to the appointed goal. For this “late Barth,” which I now am, it is indeed too late to do this in worthy fashion; he begs understanding and forgiveness. 
Barth's Letter to Emil Brunner
Barth and Brunner were close friends, until their famous debate over Natural Revelation. Barth wrote a response titled "No!" to Brunner that effectively ended their friendship. Barth and Brunner were enemies for the rest of their lives. Barth was able to talk after his stroke, but it took over a month for him to regain his ability to write anything. It is fascinating, that the very first thing Barth wrote after brush with death, was a shaky letter to Emil Brunner. The letter reveals details of Barth's stroke, and that the stroke caused him to be concerned about his broken friendship with Emil Brunner. Here's the letter:
Basel, 29 January 1965. To Prof. Emil Brunner. Zurich.
We are in a small way companions in suffering, as I had a stroke on 13 Dec., and as a result was being treated in Bethesda Hospital until this morning. I could speak again after a few hours, my first intelligible word being Zacharias (Luke 1:5ff). I can read and do so with pleasure. But I can write only with difficulty, as you may see from these lines. . . . Be that as it may, thanks again, very best wishes, and sincere greetings,
KARL BARTH 
Emil Brunner died in April 1966. Barth never repaired his friendship with Brunner, but while Brunner was on his death bead, Barth sent a letter of reconciliation to Brunner. Brunner fell into a coma shortly after hearing Barth's letter and died.
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[^1] Hunsinger, G. "Book Review: Charlotte Von Kirschbaum and Karl Barth." Center for Barth Studies. Center for Barth Studies, n.d. Web. 23 June 2017. <http://barth.ptsem.edu/index.php/Book_Reviews/Book_Review/charlotte-von-kirschbaum-and-karl-barth>.
[^2] Barth, Karl. Church Dogmatics. (n.d.): n. pag. Bloomsbury Press. 2014. Web. 2014. <http://media.bloomsbury.com/rep/files/iv-4-usa.pdf>.
[^3] Barth, Karl. Karl Barth Letters 1961-1968. Trans. Geoffrey William. Bromiley. Grand Rapids, Mich: Eerdmans, 1981. 179. Print.
Related: Charlotte von Kirschbaum, Church Dogmatics, Emil Brunner, Karl Barth, Summa