The following sentence is difficult to read because the main verb is "take," which is the third-to-last word in the sentence.
168 That thou hast sought to make us break our vows,
169 Which we durst never yet, and with strain'd pride
170 To come between our sentence and our power,
171 Which nor our nature nor our place can bear,
172 Our potency made good, take thy reward.
Here is my paraphrase: "Because you have tried to make me break my vows which I have never dared to do and with puffed-up pride tried to interfere with my right to pass judgment [on Cordelia] which neither my [kingly] nature nor my [kingly] status can bear [you must] so that I can show that I really do have potency, take your reward.
Kent's "reward" is an allowance of five days to leave Lear's kingdom. If he's not gone within that five days, he dies.