In general in the employment tribunal, each party pays its own costs. You pay yours, and your employer pays its. In other words, even if you win, your employer will not be ordered to repay any of the legal costs you have incurred.
However there are some circumstances in which you can be ordered to pay your employer’s legal costs (and vice-versa). Although 'costs' orders in the employment tribunal are still quite rare, when orders are made the amounts involved can be very large.
You can be ordered to pay costs where you, or your representative, have acted "vexatiously, abusively, disruptively or otherwise unreasonably" or where the claim you brought was 'misconceived'.
Get legal advice immediately if you think you are at risk of being ordered to pay your employer's legal costs.